More: In fact, the US president is bound by the 1973 War Powers Resolution, which limits his ability to wage war without congressional approval to 60 days.

That deadline passes on May 1, 60 days after his formal notification of the war to Congress on March 2.

It will compel him to choose one of four options " seek congressional authorisation to continue the war, start winding down US involvement, buy some breathing space or ignore the law.

Some Democrats say the war is illegal and dismiss Mr Trump's justification that it was necessary for the US national interest and "collective self-defence of regional allies, including Israel".

But several Senate Republicans have made it clear that they will not support a war without congressional approval past the 60-day mark.

Brian Mast, the representative for Florida and chairman of the House foreign affairs committee, warned that the vote could be different next time after the last of five Democratic efforts to stop the offensive failed.