US President Donald Trump on Wednesday vetoed three congressional resolutions barring billions of dollars in weapons sales to US allies including Saudi Arabia and the United Arab Emirates, which are engaged in a devastating war in Yemen.

The resolutions "would weaken America's global competitiveness and damage the important relationships we share with our allies and partners," Trump said in letters to the Senate justifying blocking them.

The measures cleared Congress this month in a strong rebuke to Trump, whose administration took the extraordinary step of bypassing legislators to approve the sales in May.

US Secretary of State Mike Pompeo had said the administration was responding to an emergency caused by Saudi Arabia's arch-foe Iran. But lawmakers including some Senate Republicans said there were no legitimate grounds to circumvent Congress, which has the right to disapprove arms sales.

Critics say the arms sales would aggravate the war in Yemen, where Saudi Arabia is leading a US-backed coalition in a battle against the Iranian-supported Huthi rebels, and which the UN said has triggered the world's worst humanitarian crisis.

But Trump argued Wednesday that barring the sale of US weapons "would likely prolong the conflict in Yemen and deepen the suffering it causes."

It is the third time the president has employed his veto power since taking office.