US President Donald Trump asked for the resignation of his national security adviser, Michael Flynn, after his level of trust in Flynn eroded to the point that he felt he needed to make a change, White House spokesman Sean Spicer says.
A day after Flynn's resignation, Spicer said Mr Trump had been concerned that Mr Flynn had misled Vice President Mike Pence over his contacts with Russian officials before Mr Trump took office.
Mr Trump had been reviewing Mr Flynn's situation for a few weeks, he said.
Amid congressional calls for an investigation into the extent to which Mr Flynn discussed the possibility of lifting US sanctions on Russia with the Russian ambassador to Washington, Mr Spicer said the White House legal counsel had reviewed the situation and believed Mr Flynn's case was viewed "not as a legal issue but a trust issue".
Mr Spicer said Mr Trump was informed of Mr Flynn's conversations with Russian ambassador Sergei Kislyak and asked counsel to review the case.
"The erosion of that trust, frankly, was the issue," Mr Spicer said.
He said Mr Trump had not told Mr Flynn to discuss US sanctions on Russia with the Russian diplomat.
Meanwhile, a senior naval officer who served under Mr Trump's Defense Secretary James Mattis is the leading candidate to replace Mr Flynn.
Vice Admiral Robert Harward, who was deputy commander of US Central Command under Mr Mattis, will likely replace Mr Flynn, the officials said, speaking on condition of anonymity as the White House scrambled to contain the fallout from the abrupt departure of one of the president's top advisers.
Losing his national security adviser three weeks after taking office is an embarrassment for the new Republican president, who has made national security a top priority.
Mr Flynn, an early supporter of Mr Trump, was a strong advocate of a softer line toward Russian President Vladimir Putin and his departure from the key post could hinder Mr Trump's efforts to warm up relations with Moscow.
Democratic President Barack Obama added sanctions on Moscow in December, weeks before leaving office, in response to what his administration charged were Moscow's efforts to try to influence the 2016 presidential election in Mr Trump's favour.
US Senator John McCain, a leading Republican voice on foreign relations, said Mr Flynn's resignation raised questions about Mr Trump's intentions toward Russia.