Republican lawmaker Thomas Massie, an outspoken critic of President Donald Trump, introduced eight articles of impeachment against US Defense Secretary Pete Hegseth on Monday as pressure builds on the Trump administration over the Iran war.

The resolution proposed by the Kentucky congressman accuses Hegseth of conducting the war illegally without congressional authorization. Massie also claims Hegseth has abused his powers as the head of the Pentagon.

Massie is using a special procedure that could see the US House of Representatives vote on the resolution within two days, with the legislative chamber still dominated by Republicans.

Trump told reporters at the White House that Hegseth had done a "good job" and had "won" wars in Venezuela and Iran.

"I'll stand by him... He won two wars. He was in charge," Trump said. 

How has Hegseth responded?

Earlier on Monday, Hegseth sought to downplay rumors about the push against him.

"My team, we’re not worried about the noise," he told reporters.

Hegseth said he would hold a press conference Tuesday — his first in more than 180 days — and defended the war as necessary, even if unpopular.

Who supports impeachment?

Democratic House leader Hakeem Jeffries backed the effort Sunday.

"I'm a hard yes on the Hegseth resolution," he told ABC News. "Pete Hegseth has got to go."

Some Republicans have signaled they might support the charges. Arkansas congressman French Hill said he wanted to hear the debate before deciding.

The push follows Democrats' victories in both chambers of Congress in last week's midterm elections amid anger over the Iran war and soaring fuel prices. Republicans retain control until January 3.

Would impeachment remove Hegseth from office?

Impeachment requires a simple majority in the House and amounts to bringing formal charges. Removal would require a Senate trial and conviction by two-thirds of senators present.

Even with their enlarged majority next year, Democrats would need substantial Republican backing to reach that threshold.