US President Donald Trump has lashed out at Democrats over their demands for US Attorney General Jeff Sessions to resign, calling their actions a "total witch-hunt".

Sessions on Thursday announced that he would recuse himself from any probe into the presidential election campaign as the White House moved to forestall a snowballing controversy over its ties to Russia.

Following newly revealed meetings he held with Russia's ambassador before the election, Sessions denied any impropriety or that he lied about those encounters in his Senate confirmation hearing.

The Republican US president declared his "total" confidence in Sessions - while adding he "wasn't aware" of contacts between Ambassador Sergey Kislyak and Sessions, who was a senator actively supporting Trump's campaign at the time.

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He defended Sessions again in a statement calling Sessions an "honest man" and accusing Democrats of having "lost their grip on reality" and carrying out "a total witch-hunt".

Sessions "did not say anything wrong. He could have stated his response more accurately, but it was clearly not intentional", said the president.

Russian Foreign Minister Sergey Lavrov also agreed with Trump on Friday, saying the attacks on Sessions looked like "a witch-hunt", Russia's RIA news agency reported.

Unswayed by Sessions' account of events, top Democrats are maintaining their calls for him to step down immediately, accusing him of perjury.

They also called for an independent prosecutor to investigate contacts between the Trump campaign and Moscow, which US intelligence says interfered in the election to hurt Trump's Democratic rival, Hillary Clinton.

Adam Schiff, a Democratic ranking member of the House Intelligence Committee, rejected Sessions' claim that his contacts with Kislyak were unrelated to his work with the Trump campaign as "simply not credible".

"In the midst of a Russian campaign aimed at undermining our election, and as a highly visible proxy for candidate Trump, Sessions would have had to be extraordinarily naive or gullible to believe that the ambassador was seeking him out in his office for a discussion on military matters, and Sessions is neither," he said in a statement.

"I have come to the reluctant conclusion that the attorney general should step down," he said, echoing calls made earlier by the top Democrats in both chambers of the Republican-controlled Congress.

Focus on Russian diplomat

Trump has come under increasing pressure over Russia's interference in the election and alleged contacts between his entourage and Moscow.

According to officials, US intelligence agencies and the FBI continue to investigate just how and how much Moscow intruded into US politics, and whether that effort - which US intelligence chiefs say was directed by President Vladimir Putin - involved collusion with the Trump campaign.

Four congressional committees have opened probes into the issue, although Democrats fear that Republicans will seek to bury their investigations to protect Trump's young administration.

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Two weeks ago, Trump's newly appointed national security adviser Michael Flynn was forced to resign amid controversy over his discussions with Kislyak in late December, when the Obama administration was hitting Moscow with retaliatory sanctions and expulsions for its election interference.

On Thursday, The New York Times reported that Flynn had also met the diplomat in Trump Tower in December, with Trump's son-in-law and senior adviser Jared Kushner also in attendance.

While meetings between political campaigns and diplomats are generally common, on Thursday a Clinton spokesman said there were no meetings or calls between her team and any Russian official during the campaign.

Sessions met Russian envoy twice

Sessions' own meetings with the envoy took place much earlier, in July and September, just as accusations of Russian interference in the election were mounting, according to The Washington Post.

However, Sessions told his confirmation hearing in January he "did not have communications with the Russians" and did not know of any by other campaign staff.

Sessions on Thursday clarified that his denial referred to contacts made on behalf of the campaign. He said he met Kislyak in his capacity as a senator, and discussed mainly global politics with him.

Nevertheless, after reviewing ethics rules for his office, he said: "I have decided to recuse myself from any existing or future investigations of any matters related in any way to the campaigns for president of the United States."

Speaking to Fox News Wednesday evening, he reiterated that he did not discuss the campaign with Kislyak.

"When I campaigned for Trump, I was not involved with anything like that," he said. "You can be sure."

Independent probe

Despite the swirling controversy, few hard facts are publicly available on what US investigators know and suspect about the contacts and about Russia's alleged operation to disrupt the election last year.

The New York Times reported two weeks ago, citing US intelligence sources, that three Trump campaign staff had communicated with Russian intelligence officers over the past year.

The White House labelled that report "false" and has accused Democrats, the media and the intelligence community of a political effort to undermine the Trump administration.

Trump's White House has lobbied the FBI, reportedly the CIA and two Republicans who head committees leading investigations into Russia's election meddling, to knock down media reports on the alleged links.

That added to Democrats' worries that investigations could be tainted, and politicians were pressing for an independent counsel to be named to study the web of allegations about Trump and Russia free from political interference.