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

The reassignment of personnel, around June, came 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 its base in Bahrain.

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

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

But 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" in Bahrain.

That number is down from about 45, in Bahrain and Saudi Arabia, at its peak.

"If the need arises," the team directly assigned to coalition cooperation could be augmented, the spokesman said.

The joint cell was established around the start of coalition operations in March last year, 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 Shiite Huthi rebels and their allies overran much of Yemen.

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

- No 'blank check' -

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.

Human Rights Watch said Friday that US Secretary of State John Kerry should raise concerns with Saudi Arabia about "repeated violations of the laws of war by the Saudi-led coalition in Yemen that have killed many civilians".

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

A US Defence Department spokesman on Saturday said that Washington's support to the coalition was not a "blank check".

"The cooperation that we've extended to Saudi Arabia since the conflict escalated again is modest and it is not a blank check," Adam Stump said.

"At no point did US military personnel provide direct or implicit approval of target selection."

But Stump said the US was "concerned by the threat to Saudi territory along its southern border with Yemen".

The coalition stepped up air strikes this month after UN-mediated peace talks between the rebels and Yemen's internationally-backed government were suspended.

The rebels have retaliated with cross-border attacks into Saudi Arabia.

Rockets fired by Yemeni rebels into the southern Saudi city of Najran on Saturday killed a civilian, the kingdom's civil defence agency said.

Seven civilians were killed in shelling in the same city last Tuesday.

- Targets chosen by Saudis -

Meanwhile, the rebel-controlled Saba news agency reported a wave of coalition raids on Saturday, including one that killed three civilians near the Huthi-held capital Sanaa.

Despite the reported attacks, Saba said that "thousands" of people demonstrated in Sanaa in support of the rebels and their allies, forces loyal to ousted president Ali Abdullah Saleh.

MSF decided to withdraw staff from six hospitals in Yemen after 19 people died in an air raid last Monday on a hospital supported by the medical charity in the 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 as well as an air raid two days earlier on a Koranic school that MSF said killed 10 children.

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

Intelligence is still being provided to the Saudis, the Fifth Fleet spokesman said.

On the issue of targets and civilian casualties, the Saudis are the ultimate decision-makers, he said. "The final decision on targets is up to them."