- Video report by ITV News Washington Correspondent Robert Moore
US President Donald Trump's national security adviser Michael Flynn has resigned after reports he misled Vice President Mike Pence and other officials about his contacts with Russia.
In a resignation letter, Flynn said he held numerous calls with the Russian ambassador to the US, Sergey Kislyak, before Trump took office and gave "incomplete information" about those discussions to Vice President Mike Pence.
The vice president, reportedly relying on information from Flynn, had publicly said the national security adviser had not discussed sanctions with the Russian envoy.
Flynn later conceded the issue may have come up.
News of his resignation, less than a month after Trump's inauguration, has caused consternation in the US while politicians in Russia said that Trump was being manipulated or over-paranoid.
The Democrats demanded a special investigation into possible links between the president's administration and Russia, saying the American people "deserve to know the full extent of Russia's financial, personal and political grip on President Trump".
There was also harsh criticism came from the senior Republican John McCain, who is chairman of the Senate Armed Services Committee.
He said that Flynn's departure "is a troubling indication of the dysfunction" of the current national security arrangements in government and raises further questions over Trump's stance towards Moscow.
He said that Flynn's successor must have the "skills and experience necessary" and the president must continue to hold Russia accountable for human rights violations and their annexation of Crimea from Ukraine.
Meanwhile House speaker Paul Ryan praised Trump for seeking Flynn's departure and said the more details about the exact circumstances were expected to be made public.
Trump himself has not publicly addressed the resignation other than to say the "real story" is leaks from Washington in a tweet.
In Russia, politicians have derided the departure of Mr Flynn while the Kremlin continued to deny Russia had discussed the lifting of sanctions with the US.
Prominent Russian politician Konstantin Kosachev condemned Flynn's resignation in a Facebook post, saying either Trump has been "backed into a corner" or his administration has been "infected with anti-Russian sentiment".
Kosachev, who chairs the foreign affairs committee in the Russian parliament's upper chamber, wrote that the national security adviser's resignation for having "contacts" with the Russian ambassador - which he said was "normal diplomatic practice" - is "not even a little paranoid, but something far worse".
Kosachev's counterpart in the Russian parliament's lower chamber, Alexei Pushkov, tweeted shortly after the announcement that "it was not Flynn who was targeted but relations with Russia".
The Russian Embassy in the US meanwhile cited a spokesperson for the Kremlin who reiterated the denial of sanction discussions.
Trump has named retired Lt. Gen. Keith Kellogg as the acting national security adviser while a shortlist is drawn up for the permanent successor.
Flynn's exit is the latest destabilising blow to an administration that has already suffered a major legal defeat.
The Trump team's account of Flynn's discussions with the Russian envoy Kislyak changed repeatedly over several weeks.
It also emerged the Justice Department warned last month Flynn could be at risk of blackmail as a result of the contradictions between the public depictions of the calls and what intelligence officials knew to be true based on recordings of the conversations.
It was unclear when Trump and Pence learned about the Justice Department warning.
The White House had indicated Flynn's future was in doubt when spokesman Sean Spicer said Trump was "evaluating the situation".
Spicer said the president was "absolutely not" aware that Flynn might have discussed sanctions with Kislyak.
The telephone conversations raised doubts as to whether Flynn, who had close ties to Russia, had offered assurances about the incoming administration's new approach.
In 2015, Flynn was paid to attend a gala dinner for Russia Today, a Kremlin-backed television station, and sat next to Russian President Vladimir Putin during the event.
Flynn's temporary replacement Kellogg had previously been appointed the National Security Council chief of staff and advised Trump on national security issues during the campaign.