The US military has slashed the number of intelligence advisers directly supporting the Saudi-led coalition's air war in Yemen, the US Navy has said.

The reassignment of personnel, around June, occurred because "there was not the same sort of requests coming in for assistance" from the Saudis, Fifth Fleet spokesman Lieutenant Ian McConnaughey told AFP from Bahrain.

Saudi Arabia has faced repeated criticism from rights groups over civilian casualties in its 17-month-long campaign against rebels in Yemen.

US officials have regularly urged their major Middle East ally to avoid harming non-combatants in Yemen.

But Lt McConnaughey said the US reassignment of personnel does not affect their ability to support the Saudis and is a more efficient allocation of resources.

"That's the main reason behind it, and it's based on the amount of requests that we receive from the Saudis."

He said the United States now has "a limited number, less than five, that are working directly on the advisory cell that we have here."

That number is down from about 45, in Bahrain and Saudi Arabia, at its peak. The joint cell was established around the start of coalition operations in March last year, Lt McConnaughey said.

The Arab coalition began air raids and later sent in ground forces to support the internationally recognised government of President Abedrabbo Mansour Hadi after Huthi rebels and their allies overran much of Yemen.

Saudi Arabia says the rebels are backed by its Shiite regional rival Iran.

The coalition has told AFP that it uses highly accurate laser and GPS-guided weapons – many of them supplied by the US – and that it verifies targets many times in order to avoid civilian casualties.

Yet allegations of strikes on civilian facilities have continued.

On Thursday Paris-based Doctors Without Borders (MSF) accused the coalition of "indiscriminate bombings" and said it had lost confidence in the alliance's ability to prevent fatal attacks on its facilities.

MSF decided to withdraw staff from six hospitals after 19 people died in an air strike on Monday on an MSF-supported hospital in Yemen's rebel-held northern province of Hajja.

That was the fourth and deadliest attack yet on an MSF facility during the war, the charity said.

US State Department spokeswoman Elizabeth Trudeau expressed deep concern after reports of the hospital strike.

A coalition investigative team is conducting "independent" probes into the hospital strike and an air raid two days earlier on a Koranic school which MSF said killed ten children.

Lt McConnaughey said the US cooperation with Saudi Arabia mostly involves "imagery that allows them to better assess the situation on the ground, and then advice and assistance."

He said intelligence is still being provided to the Saudis.

"The only difference is it's based on the demand signal. We're not going to basically have a bunch of people available if there's no immediate request for them."

The spokesman for the coalition could not be immediately reached.