President Donald Trump on Tuesday vetoed a bill passed by Congress to end US military assistance in Saudi Arabia’s war in Yemen.

In a break with the president, Congress voted for the first time earlier this month to invoke the War Powers Resolution to try to stop US involvement in a foreign conflict.

The veto - the second in Trump’s presidency - was expected. Congress lacks the votes to override him.

"This resolution is an unnecessary, dangerous attempt to weaken my constitutional authorities, endangering the lives of American citizens and brave service members, both today and in the future," Trump wrote in explaining his veto.

Congress has grown uneasy with Trump’s close relationship with Saudi Arabia as he tries to further isolate Iran, a regional rival.

Many lawmakers also criticised the president for not condemning Saudi Arabia for the killing of Jamal Khashoggi, a Saudi who lived in the United States and had written critically about the kingdom.