WASHINGTON, Jan. 11 (CNA) - Acting Homeland Security Secretary Chad Wolf is stepping down from his post, days after criticising President Donald Trump over the riot at the US Capitol.

Wolf, whose department is in charge of security for President-elect Joe Biden's inauguration on Jan 20, will step down at midnight, according to a letter he sent to DHS staff.

The announcement makes him the third person to resign from Donald Trump's Cabinet after the attack by the president's supporters on the US Capitol last week.

Wolf said he had intended to stay in the post until the inauguration of President-elect Joe Biden.

But Wolf, who had been serving in an acting capacity since November 2019 and was never confirmed by the Senate, said he was compelled to leave by “recent events", including court rulings that found he could not legally hold the position. He did not specify the other events or cite other factors.

“These events and concerns increasingly serve to divert attention and resources away from the important work of the Department in this critical time of a transition of power,” he said in a written message to DHS employees.

The departure of Wolf - long a close ally of Trump - follows the abrupt resignation of other Cabinet officials angered by Trump's role in encouraging the mob to storm the Capitol on Jan 6 over his false claims of election fraud.

"While I have consistently condemned political violence on both sides of the aisle, specifically violence directed at law enforcement, we now see some supporters of the president using violence as a means to achieve political ends," he said.

The White House had no immediate comment.