National Security Advisor Michael Flynn has resigned over controversy surrounding his ties with Russia.

The military veteran stepped down late on Monday night less than a month into President Donald Trump's administration amid mounting questions over his future and his close links with the Kremlin.

His stunning announcement, first reported by CNN, came just hours after sources close to the government revealed that the Justice Department had warned the new administration that his links with the Kremlin had put him in a compromising position and left him open to blackmail.

Flynn made numerous phone calls to the Russian Ambassador to the US before Trump took office, and later hinted he may have gone against diplomatic protocol by discussing the rollback of sanctions.

National Security Advisor Michael Flynn resigned on Monday night over controversy surrounding his contacts with Russia

Flynn (center) stepped down just 24 days into the Trump Administration

Flynn's resignation came just hours after sources close to the government revealed that the Justice Department had warned the new administration that his links with the Kremlin had put him in a compromising position and left him open to blackmail

Vice President Mike Pence vouched for Flynn, without knowing the full details of the calls, and gave him his full support. Flynn has now issued a grovelling apology to Pence.

Retired general Keith Kellogg will take over as acting National Security Advisor.

Former CIA Director David Petraeus and US Navy Admiral Michelle Howard are among the names being touted as a replacement.

TIMELINE OF FLYNN'S RUSSIA TROUBLES December 29: Obama announces sanctions against Russia over alleged hacks targeting election. Flynn spoke with Russian ambassador Kislyak repeatedly the same day, it was later reported. December 30: Russian President Vladimir Putin says he will not respond to the sanctions, surprising many U.S. officials. Trump praises Putin's decision as 'very smart.' January 13: Trump spokesman Sean Spicer says Flynn had called Kislyak merely to set up a phone call between Trump and Putin January 15: Vice President-elect Mike Pence says in an interview with CBS that Flynn had not discussed santions in the call with Kislyak January 20: Trump inaugurated president January 23: Spicer, now White House spokesman, said Flynn had told him the call to Kislyak covered four topics: a plane crash that killed a Russian military choir; Christmas greetings; Russian-led talks over the Syrian civil war; and logistics for a call between Putin and Trump January 20-30 (exact date unclear): Acting Attorney General Sally Yates briefs Trump officials that intercepted communications indicated Flynn had discussed sanctions with Kislyak in ways she believed were 'highly significant' and 'potentially illegal' January 30: Trump fires Yates for refusing to defend executive immigration order in court February 8: Flynn denies in interviews that he discussed sanctions in the calls with Kislyak February 9: Flynn backpedals, saying through a spokesperson that he 'couldn't be certain' sanctions weren't discussed February 13: Report breaks of Justice Department concerns about blackmail, and Flynn resigns hours later Advertisement

In his resignation letter, Flynn wrote: 'I inadvertently briefed the Vice President Elect and others with incomplete information regarding my phone calls with the Russian Ambassador. I have sincerely apologized to the President and the Vice President, and they have accepted my apology.'

Democrats in the House of Representatives requested a classified briefing within hours of the abrupt resignation announcement.

Reps. John Conyers Jr (D-Mich.) and Elijah Cummings (D-Md.), respectively the ranking members of the Judiciary and Oversight committees said in a statement: 'We were shocked and dismayed to learn this evening of reports that three weeks ago, U.S. law enforcement officials warned the White House Counsel that General Flynn had provided false information to the public about his communications with the Russian government.

'The reality is General Flynn was unfit to be the National Security Advisor, and should have been dismissed three weeks ago.'

'We need to know who else within the White House is a current and ongoing risk to our national security,' the statement reported in The Hill said.

The Justice Department weeks ago warned the Trump administration about Flynn's contacts with Russia, according to a report earlier Monday.

Government officials supposedly informed the White House because they were concerned Flynn could be vulnerable to blackmail by the Kremlin.

Former acting attorney general Sally Yates told the new administration last month she was concerned over potential blackmail after Flynn denied that he had discussed sanctions in calls with the Russian ambassador to the United States, the Washington Post reported just hours before the resignation.

Intelligence intercepts of those calls indicate that they did touch on sanctions in ways Yates believed were 'highly significant' and 'potentially illegal', leaving the Kremlin with potential leverage over Flynn, the Post reported.

Flynn, a former director of the Defense Intelligence Agency, spoke to Russian Ambassador Sergey Kislyak more than once before Trump took office, sparking concerns that the two were discussing US policy toward Russia.

One call took place on the same day that President Obama applied sanctions against Moscow for their alleged interference in the presidential election.

A Trump administration official says the White House was aware of the warning, but it's unclear whether Trump himself was briefed on the matter.

Flynn made numerous phone calls to the Russian Ambassador to the US before Trump took office, and later hinted he may have gone against diplomatic protocol by discussing sanctions

FLYNN'S RESIGNATION LETTER In the course of my duties as the incoming National Security Advisor, I held numerous phone calls with foreign counterparts, ministers, and ambassadors. These calls were to facilitate a smooth transition and begin to build the necessary relationships between the President, his advisors and foreign leaders. Such calls are standard practice in any transition of this magnitude. Unfortunately, because of the fast pace of events, I inadvertently briefed the Vice President Elect and others with incomplete information regarding my phone calls with the Russian Ambassador. I have sincerely apologized to the President and the Vice President, and they have accepted my apology. Throughout my over thirty three years of honorable military service, and my tenure as the National Security Advisor, I have always performed my duties with the utmost of integrity and honesty to those I have served, to include the President of the United States. I am tendering my resignation, honored to have served our nation and the American people in such a distinguished way. I am also extremely honored to have served President Trump, who in just three weeks, has reoriented American foreign policy in fundamental ways to restore America's leadership position in the world. As I step away once again from serving my nation in this current capacity, I wish to thank President Trump for his personal loyalty, the friendship of those who I worked with throughout the hard fought campaign, the challenging period of transition, and during the early days of his presidency. I know with the strong leadership of President Donald J. Trump and Vice President Mike Pence and the superb team they are assembling, this team will go down in history as one of the greatest presidencies in U.S. history, and I firmly believe the American people will be well served as they all work together to help Make America Great Again. Advertisement

Earlier on Monday, conflicting messages about Flynn's fate criss-crossed cable TV from different corners of the West Wing.

First, Chief White House counselor Kellyanne Conway told MSNBC that President Donald Trump wasn't planning to axe Flynn over accusations that he lied to Vice President Mike Pence about his conversations with Russia.

'General Flynn does enjoy the full confidence of the president,' Conway said.

I inadvertently briefed the Vice President Elect and others with incomplete information regarding my phone calls with the Russian Ambassador. -FLYNN RESIGNATION LETTER

Less than an hour later, press secretary Sean Spicer said in a statement that the president 'is evaluating the situation'.

'He's speaking to Vice President Pence relative to the conversation the Vice President had with Gen. Flynn,' Spicer said, 'and also speaking to various other people about what he considers the single most important subject there is: our national security.'

Spicer told reporters in his office on Monday evening that Flynn has not offered to quit his position, and called questions about whether Trump would accept his resignation 'hypothetical.'

He said he had spoken directly with Trump about the latest statement's wording, declaring that it reflected the president's 'current thinking.'

'This is what he asked me to communicate to you,' Spicer recounted.

After flatly denying he'd discussed rolling back sanctions with Russian officials in a January 15 interview, Flynn said he was uncertain whether the subject came up.

Two administration officials told DailyMail.com on Friday that Flynn was still unsure what he told Kislyak, the Russian ambassador. One said he couldn't be '100 percent' certain sanctions were not discussed.

Monday's joint press conference between Trump and Canadian Prime Minster Justin Trudeau came and went without a single Flynn-related question.

Retired General Keith Kellogg will take over as acting National Security Advisor

Trump didn't respond to shouted queries as he left the White House's East Room.

As of Monday night, he had not commented on Flynn's departure.

Hillary Clinton took to Twitter on Monday night to share her opinion on National Security Advisor Michael Flynn's resignation, suggesting there are 'real consequences of fake news'.

Clinton retweeted a post from longtime confidante Phillipe Reines on Monday night, where he suggested Flynn and his son, who spread the discredited 'Pizzagate' conspiracy theory on social media, get jobs at Domino's.

'Philippe's got his own way of saying things, but he has a point about the real consequences of fake news,' Clinton said in her tweet.

Reines had written, 'Dear Mike Flynn & Mike Flynn Jr., What goes around COMETS around. And given your pizza obsession...', with a link to Domino's Pizza's job site.

In December, Flynn Jr tweeted about the Pizzagate conspiracy, which linked Clinton and her campaign chief to Comet Ping Pong pizza shop in Washington, DC.

The theory suggested that Clinton and John Podesta presided over a child-sex ring run out of the DC pizza parlor.

Hillary Clinton retweeted a post from longtime confidante Phillipe Reines on Monday night after retired Army Lieutenant General Michael Flynn resigned from his position as National Security Advisor

Reines suggested in his tweet that Flynn and his son, who spread the discredited 'Pizzagate' conspiracy theory on social media, get jobs at Domino's

In his tweet, Flynn Jr did not endorse the conspiracy theory, but predicted that it wouldn't go away

WHO WILL REPLACE FLYNN AS NATIONAL SECURITY ADVISOR? Retired Army General Keith Kellogg, a member of Trump's transition team, will temporarily fill the role of National Security Advisor until Michael Flynn's announced. Murmurs of who will take over the position have circulated and include the potential for Kellogg to stay on permanently. At least three other candidates have been discussed with the media, including retired Navy Vice Admiral Robert Harward, Former CIA Director David Petraeus and former US Navy Admiral Michelle Howard. Retired Lieutenant General Keith Kellogg, Interim National Security Advisor A senior administration official reportedly told NBC News that Kellogg is being considered to stay on permanently. Kellogg, 72, was previously appointed by Trump as the head of the presidential transition agency action team for defense. He was a former commander of the 82nd Airborne Division, and served as chief operating officer of the Western coalition in Baghdad, Iraq, after the United States led the invasion in 2003. Retired Army General Keith Kellogg, a member of Trump's transition team, will temporarily fill the role of National Security Advisor, and there are rumors he may stay in the role permanently Former CIA Director David Petraeus Former CIA director and retired Army General David Petraeus is also reportedly being discussed as a potential to fill Flynn's shoes. National Security Council staffers say Petraeus is due to meet with President Trump at the White House this week, according to the Huffington Post. Petraeus, a retired four-star general, was bounced from his position atop the intelligence agency in 2012 after he it was revealed that he passed on classified information to his biographer, who had also become his mistress. Petraeus reportedly ended the affair with his principal biographer Paula Broadwell in 2012 after learning that she had sent threatening emails to a long-time family friend, Jill Kelley. He resigned from his position with the CIA in November 2012 and later pleaded guilty on a misdemeanor charge for mishandling classified materials. Petraeus was briefly under consideration to become Secretary of State before Trump picked Exxon Mobil CEO Rex Tillerson. Former CIA director and retired Army General David Petraeus is also reportedly being discussed as a potential to fill Flynn's shoes Retired Vice Admiral Robert Harward Another name being discussed is retired Vice Admiral Robert Harward, the former deputy to Defense Secretary James Mattis. Additionally, he is a retired Navy SEAL and served as the Deputy Commander of US Joint Forces Command. Upon retirement in 2013 after a nearly 40-year career in the Navy, Harward took a post as a chief executive officer for defense and aerospace giant Lockheed Martin in the United Arab Emirates. Another name being discussed is retired Vice Admiral Robert Harward, the former deputy to Defense Secretary James Mattis Trump has recently been in very public negotiations with Lockheed over the cost of its F-35 fighter jet program. Two senior US officials reportedly confirmed to NBC News that Harward was the 'favorite' for the position. US Navy Admiral Michelle Howard US Navy Admiral Michelle Howard has also been said to be under consideration. She is the current commander of US Naval Forces in Europe, and was the first African-American woman to command a US Navy ship and hold the position of Vice Chief of Naval Operations. Among her other record-setting achievements, she was also the first female graduate of the Naval academy to be selected for flag rank. Former National Security aide Tom Bossert Tom Bossert, President Trump's choice for Homeland Security Advisor, was also mentioned by Karam as a candidate for the position. Bossert served as a national security aide to George W Bush during his last year as president. He also served two years as a White House director of infrastructure policy. Retired Admiral James Stavridis Though James Stavridis previously said that he would not serve under the Trump administration, his name has come up amongst potentials for NSA. He currently serves as the Dean for Tufts University's Fletcher School of Law and Diplomacy. When asked in 2016 about the potentiality of him joining the Trump team, he said: 'I'm happy to provide advice, but I don't see myself joining the administration.' US Homeland Security Secretary John Kelly John Kelly is the current US Homeland Security Secretary appointed by Donald Trump, but has reportedly been discussed as Flynn's replacement. Kelly recently made headlines after telling San Diego Police Chief Shelley Zimmerman that he didn't 'have a clue' what a sanctuary city was. Kelly is a retired four-star United States Marine Corps General. Advertisement

Police said a 28-year-old North Carolina man opened fire with a rifle inside the restaurant in December because he thought child sex-slaves were being held against their will in the basement.

No one was harmed but the man was arrested.

In his tweet, Flynn Jr did not endorse the conspiracy theory, but predicted that it wouldn't go away.

'Until #Pizzagate proven to be false, it'll remain a story. The left seems to forget #PodestaEmails and the many 'coincidences' tied to it,' he tweeted.

Meanwhile, Russian lawmakers suggested that Flynn had been forced out of his position because Trump’s administration had been 'infected' by anti-Russian feeling.

'Either Trump has not gained the requisite independence and he is consequently being not unsuccessfully backed into a corner, or Russophobia has already infected the new administration also from top to bottom,' MP Konstantin Kosachev was cited as saying by the RIA Novosti news agency.

Kosachev, chairman of the foreign affairs committee at the upper chamber of the Russian parliament, said in a post on Facebook that firing a national security advisor for his contacts with Russia is 'not just paranoia but something even worse'.

Kosachev's counterpart at the lower chamber of the Russian parliament, Alexei Pushkov, said shortly after the announcement that 'it was not Flynn who was targeted but relations with Russia'.

Leonid Slutsky, head of the lower house of parliament's foreign affairs committee, suggested that Flynn was forced out of office.

MP Konstantin Kosachev (left) said Flynn's resignation from national security advisor was a sign of 'not just paranoia but something even worse' in the White House. MP Alexei Pushkov (right) said shortly after the announcement that 'it was not Flynn who was targeted but relations with Russia'

Leonid Slutsky, head of the lower house of parliament's foreign affairs committee, suggested that Flynn was forced out of office

'It's obvious that Flynn was forced to write the letter of resignation under a certain amount of pressure,' he was quoted as saying by the RIA news agency.

'The target was Russia-U.S. relations, undermining confidence in the new U.S. administration. We'll see how the situation develops further,' he said.

Meanwhile, Kremlin spokesman Dmitry Peskov said on Tuesday that the resignation was an internal matter for the United States.

"We've said everything we want to say," Peskov told reporters on a conference call.

Peskov has previously said that Flynn and the ambassador did not discuss lifting sanctions in their conversations.

He declined to elaborate on those earlier comments when asked on Tuesday.

Flynn's military career ended when Obama dismissed him as defense intelligence chief. He claimed he was pushed out for holding tougher views than the Obama administration about Islamic extremism.

But a former senior US official who worked with Flynn said the firing was for insubordination, after the Army lieutenant general failed to follow guidance from superiors.

Once out of government, he disappeared into the murky world of mid-level defense contractors and international influence peddlers.

He shocked his former colleagues a little more than a year later by appearing at a Moscow banquet headlined by Russian President Vladimir Putin.

Given a second chance by Trump, Flynn, a lifelong if apolitical Democrat, became a trusted and eager confidant of the Republican candidate, joining anti-Hillary Clinton campaign chants of 'Lock Her Up' and tweeting that 'Fear of Muslims is RATIONAL'.

As national security advisor, Flynn required no Senate confirmation vote or public vetting of his record, and his tenure was brief but turbulent.