National security adviser Michael Flynn has resigned after claims he misled the Trump administration over his talks with Russia.
In his resignation letter, he admitted he had "inadvertently briefed the vice president-elect and others with incomplete information regarding my phone calls with the Russian ambassador".
Mr Flynn said he had "sincerely apologized to the President and the Vice President, and they have accepted my apology".
It comes amid claims he had discussed sanctions with Moscow's US envoy before Mr Trump was in power - and before he had officially taken up his own role.
Any attempt to conduct diplomacy on behalf of the United States while a private citizen could be illegal.
Mr Flynn initially told Vice President Mike Pence he had not discussed sanctions during the meeting with the ambassador, prompting Mr Pence to defend him in a TV interview.
In recent days however, he admitted he might have raised the subject of sanctions with the Russians but said he could not be 100% certain.
President Obama imposed sanctions on Russia over its annexation of Crimea and claims it tried to influence the presidential election.
Mr Flynn's resignation comes hours after the White House said it was "evaluating" the situation, and following Mr Trump's close aide, Kellyanne Conway, telling reporters the security adviser's job was safe.
Retired lieutenant general Joseph Kellogg will take up the role until a permanent successor is appointed.
The former CIA director and retired general, David Petraeus, is under consideration for the role, according to a White House official.
A US official has also confirmed a Washington Post
report that the nation's acting attorney general had warned the White House last month that Mr Flynn could be vulnerable to Russian blackmail over his early contact with the ambassador.
(Additional reporting from IRN)