Washington - The White House is battling to insulate Donald Trump from the scandal over a top aide's contacts with Russia, as calls grew for an independent investigation.Trump's young presidency has been thrown into turmoil after the forced resignation of his national security advisor and long-time supporter Michael Flynn.The retired three-star general and former head of US defence intelligence is accused of discussing sanctions strategy with Russia's ambassador Sergey Kislyak before taking office.The White House said that after weeks of internal investigation - which turned up no wrongdoing but "eroded" trust - Trump had requested and accepted Flynn's resignation late on Monday.Flynn is the third Trump aide to step back amid questions about his ties to Russia since the mogul began his improbable White House bid. His departure follows those of election campaign manager Paul Manafort and Carter Page, an early foreign policy advisor to the candidate.The unprecedented early resignation of a key member of staff has rocked an administration already buffeted by leaks, infighting and legal defeats.Amid the tumult, the White House denied that Trump had instructed Flynn to discuss the possibility that Obama-era sanctions would be rolled back."No, absolutely not. No, no, no," said White House spokesperson Sean Spicer, when asked whether such a conversation took place.'Eroding trust'Spicer said the president "instinctively thought that General Flynn did not do anything wrong and the White House counsel's review corroborated that," adding that the counsel "determined that there is not an illegal issue, but rather a trust issue.""The evolving and eroding level of trust as a result of this situation and a series of other questionable instances is what led the president to ask for General Flynn's resignation."Trump took to Twitter to insist that "The real story here is why are there so many illegal leaks coming out of Washington?"The White House also insisted that Trump - despite repeatedly professing admiration for Vladimir Putin and suggesting sanctions could be lifted - "has been incredibly tough on Russia."In a new hardening of the US line on Russia, Spicer added that "President Trump has made it very clear he expects the Russian government to de-escalate violence in the Ukraine and return Crimea."The State Department meanwhile expressed concern that Russia is in breach of the Intermediate-Range Nuclear Forces Treaty, after reports that Moscow had deployed an operational ground-launched cruise missile unit.Inquiries and misstepsThe White House's efforts are likely to do little to assuage concerns on Capitol Hill about Russia's influence in US politics.On Tuesday the White House admitted the president had known as early as January 26 that Flynn may have made misleading statements, apparently contradicting Trump's statement last Friday that he was unaware of the issue.Republicans and Democrats in Congress have now called for an investigation into what occurred, although they differ sharply on the scope and powers."This. Is. Not. Normal." said Democratic Senator Elizabeth Warren, insisting "Trump owes Americans a full account" of his administration's dealings with Moscow before and after the 2016 election.The US Senate's top Republican Mitch McConnell said it was "highly likely" that Flynn would have to testify before an intelligence panel, potentially heaping pressure on Trump.The CIA, FBI and other intelligence agencies have already investigated Moscow's influence over the 2016 vote, concluding the Kremlin tried to sway it in Trump's favour.Various committees in the Republican-controlled Congress are looking into Russia's election-related hacking and the Trump campaign's links to Moscow.Possible replacementsFlynn's resignation came after details of his calls to the ambassador were made public - increasing pressure on Trump to act.The Justice Department had warned the White House that Flynn misled senior administration officials about the contents of his talks with Kislyak, and that it could make him vulnerable to Russian blackmail, US media reported.The message was delivered in the last days of Barack Obama's administration by then-acting attorney general Sally Yates - who Trump sacked after she instructed government lawyers not to defend the new president's controversial travel ban.Until quitting, Flynn had been instrumental in Trump's inner circle.He was an early supporter of his improbable bid for the presidency and had encouraged tougher policies on Iran and a softer line on Russia.That was a sharp break from the Obama administration, which introduced sanctions over Moscow's annexation of Ukraine's Crimea, support for separatists in eastern Ukraine, and what US intelligence says were its attempts to sway last year's election in Trump's favour.Washington and Moscow had also clashed over alleged war crimes in Syria, where Russia is accused of aiding the bombing of hospitals and other civilian targets. Despite this, Flynn had argued for rapprochement.After Flynn quit, the White House said Trump had named retired lieutenant general Keith Kellogg, a decorated Vietnam war veteran who was serving as a director on the Joint Chiefs of Staff, to be interim national security advisor.Potential permanent replacements reportedly include three retired military brass: Kellogg; retired general and former CIA director David Petraeus; and former vice admiral Robert Harward.