The Senate is next week slated to vote on a measure that would curtail US involvement in Yemen's bloody civil war, where the Saudi-supported government is fighting Iranian-backed Huthi rebels.

Some US lawmakers have long expressed concern about the conflict, which has seen high levels of civilian casualties and caused a humanitarian crisis.

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

Critics say Riyadh would be unable to conduct much of its campaign without US help.

In a letter to congressional leaders this week, Mattis said that restricting US support to the campaign could lead to additional harm on the ground, because US targeting and intelligence are key to reducing the civil toll.

"New restrictions on this limited US military support could increase civilian casualties, jeopardize cooperation with our partners on counterterrorism, and reduce our influence with the Saudis -- all of which would further exacerbate the situation and humanitarian crisis," Mattis wrote.

Speaking to reporters accompanying him back to Washington after a visit to the Middle East, Mattis said Thursday he sees the current path as helping push the Yemen crisis to a UN-brokered peace deal.

"We need to get this to a negotiated settlement and we believe the policy right now is correct, and that was the gist of my letter," Mattis said.

He wrote that withdrawing US support to the Saudi-led coalition would have knock-on effects resulting in deeper Iranian involvement in the war.

It would enable "further ballistic missile strikes on Saudi Arabia and (threaten) vital shipping lanes, thereby raising the risk of a regional conflict," Mattis stated.

A bipartisan group of senators including Bernie Sanders is pushing for the Senate vote, which could come just as Saudi Crown Prince Mohammed bin Salman is visiting Washington.