Democrat senator Elizabeth Warren has hit out at President Donald Trump over his military action against Assad's regime in Syria.

Warren suggested Trump's reasoning on Syria was 'erratic' and said the president should help the Syrian people by dropping his plan to ban refugee children from entering the US.

Explaining his decision to fire missiles at an air base in Homs as a response to the horrific Khan Sheikhoun chemical attack, Trump said that the strike was in the "vital national security interest" of the US.

"Years of previous attempts at changing Assad's behaviour have all failed and failed very dramatically," he said.

"As a result the refugee crisis continues to deepen and the region continues to destabilise, threatening the US and its allies."

But in a series of Tweets, Warren hit out at the president, writing: "We need a plan to hold Assad accountable. But so far I've seen no compelling strategic justification for Trump's missile strike.

"Within a week, Trump's Admin went from asserting they wouldn't intervene to launching Tomahawk missiles against Assad. That's erratic.

"The President's erratic approach to Syria is not a strategy. It underscores why our Constitution doesn't let Presidents wage war alone.

She added: "Congress must demand an actual strategy, clear goals, and a plan for achieving them before approving additional military action in Syria.

"If Trump truly wants to help Syrians fleeing murderers, he should drop his effort to ban their children from America immediately."

The Massachusetts senator has a long history of clashing with the Republican president – with the pair sparring on social media during Trump's presidential campaign.