A Republican lawmaker is seeking to impeach US Defence Secretary Pete Hegseth, accusing him of violating the Constitution by ordering military action against Iran without congressional approval.

Representative Thomas Massie says Hegseth broke the law by continuing military operations in Iran for more than 90 days without authorisation from Congress.

Massie has introduced eight articles of impeachment against the defence secretary and plans to force a vote in the House of Representatives this week.

Massie says his move is also intended to pressure President Donald Trump into replacing Hegseth before Democrats have a chance to launch their own impeachment proceedings.

Trump has strongly criticised Massie, questioning his motives and calling him “a stone-cold loser.”