US President Donald Trump's national security adviser Michael Flynn has resigned over allegations he broke the law, having inappropriate contact with Russia.
Mr Flynn had promised Russia Mr Trump would lift sanctions when in power - before he was inaugurated - according to law enforcement sources.
If Mr Flynn had had those discussions he would have violated the Logan Act against interference in foreign diplomacy by non-government officials.
US Vice President Mike Pence defended Mr Flynn in televised interviews, saying he had spoken with Mr Flynn, who assured him he had not spoken about sanctions with the Russian ambassador.
But an investigation said Mr Flynn misled Pence, prompting the White House to issue a statement saying Mr Trump was "evaluating the situation" around Mr Flynn.
Mr Flynn has now issued a statement saying he "inadvertently" briefed Mr Pence with "incomplete information regarding my phone calls with the Russian Ambassador".
The White House confirmed Mr Flynn resigned on Monday night and retired Lieutenant General Joseph Keith Kellogg, Jr has been named as his acting replacement.
"Prior to his retirement, General Kellogg was Director of the Command, Control, Communications, and Computers Directorate under the Joint Chiefs of Staff," the White House said.
The US Justice Department had warned the Trump administration last month that Mr Flynn was vulnerable to blackmail by Russia.
"I have sincerely apologised to the President and the Vice President, and they have accepted my apology," Mr Flynn said.
Mr Flynn had only been in the position for three weeks.