President Donald Trump with Russia's ambassador to the US, Sergei Kislyak. Russian Embassy National security experts reacted with alarm Monday night to reports that President Donald Trump disclosed classified information to Russia's foreign minister and ambassador to the US during a meeting in the Oval Office last week.

"This story is nauseating," Benjamin Wittes, a senior fellow in Governance Studies at The Brookings Institution, posted on Twitter along with a link to The Washington Post's bombshell story. "You might have to work with natsec," or national security, "people to understand how bad it is, but it's horrible. Really really bad."

The Post reported on Monday night that Trump's disclosures to Russian Foreign Minister Sergei Lavrov and Ambassador Sergei Kislyak "jeopardized a critical source of intelligence on the Islamic State" and "had been provided by a US partner through an intelligence-sharing arrangement considered so sensitive that details have been withheld from allies and tightly restricted even within the US government."

Many have noted that Trump, as president, is legally allowed to essentially disclose classified information to whomever he wants. But the fact that the information he shared was not a US secret, but that of an American ally, may complicate his authority to declassify information at will. Matthew Rosenberg of The New York Times told CNN on Monday that the intel came from a close "Middle Eastern ally."

"This is appalling," said Eliot Cohen, a professor at Johns Hopkins University's School of Advanced International Studies and former counsel to Secretary of State Condoleezza Rice. "If accidental, it would be a firing offense for anyone else. If deliberate, it would be treason."

"It's so mind-boggling, I don't even know what to say," Eric Edelman, who served as undersecretary of defense during the George W. Bush administration, told The Wall Street Journal. "I'm completely gobsmacked. It's jeopardizing a human source. It's the one thing you're trained to never do. If what Post is reporting is true, it's a stunning indication of his unfitness for office."

"This is the most serious charge ever made against a sitting president of the United States," Harvard professor and constitutional law scholar Alan Dershowitz said.

'This is not a garden variety breach'

Lawfare, a website founded and managed by national security experts and former members of the US intelligence community, published its initial reaction to The Post's story shortly after it broke.

"This is perhaps the gravest allegation of presidential misconduct in the scandal-ridden four months of the Trump administration," the authors, including former National Security Agency lawyer Susan Hennessey, wrote on Monday.

"This is not a garden variety breach, and outrage over it is not partisan hypocrisy about protecting classified information," they continued. "The information allegedly disclosed here appears to be of an extremely sensitive nature," as evidenced by top White House officials' reported attempts to "contain the potential fallout" from Trump's misstep by quickly contacting the directors of the CIA and the NSA.

Trump and Russian Foreign Minister Sergei Lavrov. Thomson Reuters

Trump "revealed more information to the Russian ambassador than we have shared with our own allies," one official told The Post. The incident happened last Wednesday, one day after Trump fired the FBI director, James Comey, amid an FBI investigation into whether the Trump campaign colluded with Russian officials to undermine Hillary Clinton in the 2016 election.

Trump's national security adviser, H.R. McMaster, told reporters on Monday night that The Post's reporting was "false" and that Trump did not disclose the "methods and sources" of US intelligence with the Russians during their meeting.

But Trump appeared to confirm on Tuesday morning that he had shared information with the Russians.

"As President I wanted to share with Russia (at an openly scheduled W.H. meeting) which I have the absolute right to do, facts pertaining to terrorism and airline flight safety," Trump tweeted. "Humanitarian reasons, plus I want Russia to greatly step up their fight against ISIS & terrorism."

Lawfare said Trump's reported actions "may well be a violation of the President's oath of office."

The authors explained:

"Questions of criminality aside, we turn to the far more significant issues: If the President gave this information away through carelessness or neglect, he has arguably breached his oath of office ... In taking the oath President Trump swore to 'faithfully execute the Office of President of the United States' and to 'preserve, protect and defend the Constitution of the United States' to the best of his ability. It's very hard to argue that carelessly giving away highly sensitive material to an adversary foreign power constitutes a faithful execution of the office of President."

Jim Jeffrey, a former US ambassador who served as deputy national security adviser in the George W. Bush administration, told Business Insider the report would be more concerning if Trump had shared those intelligence sources and methods with the Russians. But he cautioned that "the ally who provided the info will have to be told, and that could restrict future reporting" of sensitive intelligence between the partners.

Jeremy Bash, the former Pentagon chief of staff under President Barack Obama, echoed that sentiment: "Giving the Russians intelligence that our counterterrorism partners have asked us to protect is incredibly dangerous," he told The Wall Street Journal. "It will ensure that those partners don't share with us the information we need to protect ourselves."

Former CIA Deputy Director Michael Morell told CBS on Monday night that Trump's disclosures are "highly damaging, for two reasons: First, the Russians will undoubtedly try to figure out the source or the method of this information to make sure that it is not also collecting on their activities in Syria -- and in trying to do that they could well disrupt the source."

"The second damage is that countries who provide the United States with intelligence information will now have pause to do so if the United States is sharing such information with the Russians without their permission," Morell added.

Additionally, Jeffrey said, "whatever the president shared with the Russians beyond the absolute minimum raises the specter that he doesn't get how justifiably outraged Americans are at the hacking of the 2016 election. That's the most troubling."