US legislators will vote on whether to end US involvement in Yemen's devastating war as Saudi Arabia's crown prince, a driving force behind the conflict, visits Washington.

The rare Senate vote on Tuesday addressing US war powers aims to shut down US military involvement in Yemen within a month, unless Congress formally authorises continued involvement.

The US military is currently supporting a Saudi-led coalition fighting Houthi rebels in Yemen since 2015.

The vote - brought by a bipartisan group of senators, including Bernie Sanders - comes on a day when President Donald Trump met Saudi Crown Prince Mohammed bin Salman.

"If you look at the War Powers Act, what America is currently involved in constitutes a military action. That's pretty clear," Sanders said earlier this month. "If we can establish this principle, it will be a significant departure in policy for the United States."

Some US legislators have expressed concern about the three-year-old Yemen conflict, which has killed at least 10,000 people, led millions to face famine, instigated a deadly cholera outbreak, and caused the world's worst humanitarian crisis.

'Unauthorised war'

The Pentagon, since 2015, has provided "non-combat support" to Saudi Arabia, including intelligence sharing and air-to-air refueling for its warplanes.

James Mattis, the US defence secretary, last week asked Congress not to interfere with the US' role, warning restrictions could "increase civilian casualties, jeopardise cooperation with our partners on counterterrorism, and reduce our influence with the Saudis - all of which would further exacerbate the situation and humanitarian crisis".

Yemen's conflict is seen as both a civil conflict and a proxy war between regional titans Saudi Arabia and Iran.

Senators Sanders, Mike Lee, and Chris Murphy said earlier this month their resolution would force the first-ever vote in the Senate "to withdraw US armed forces from an unauthorised war".

"We believe that, as Congress has not declared war or authorised military force in this conflict, the United States involvement in Yemen is unconstitutional and unauthorised, and US military support of the Saudi coalition must end," said Sanders.

France targeted

Meanwhile, a report commissioned by human rights groups said France may have broken international law by providing weapons and technical help to Saudi Arabia and the United Arab Emirates (UAE) in their military operations in Yemen.

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The report by Paris law firm Ancile said France was in all probability continuing to export arms to Saudi and UAE with no guarantee they will not be used in Yemen.

The weapons exports would likely "constitute a violation by France" of the UN's Arms Trade Treaty and the EU's Common Position on Arms Export, said the report commissioned by Amnesty International and French anti-torture group Acat.

A foreign ministry spokeswoman insisted on Tuesday that "France has a robust and transparent system of controls on exports of weapons of war".

"Export decisions are taken under the prime minister with strict respect for France's international commitments," she said.

Saudi Arabia is a major buyer of Western weapons and European governments have come under pressure from NGOs over fears their arms could potentially be implicated in war crimes in Yemen.

Norway suspended arms exports to the UAE while Germany said no weapons will be supplied to countries involved in the war.