A deadly air strike by a Saudi-led coalition on a funeral in Yemen embarrassed Washington and undermined its efforts to pressure Moscow over similar carnage in Syria.

The United States provides intelligence, advanced munitions and logistics support to the Saudi-led war effort in Yemen and is the kingdom's biggest arms supplier.

On Saturday the White House announced an "immediate review" of this cooperation in the wake of a strike on a funeral in Sanaa that killed at least 140 people.

But Washington finds itself in a diplomatic bind: Saudi Arabia is a long-time ally and is seen as a counterweight against Iranian ambitions in its region.

It is also a key partner in US efforts to resolve the conflict in Syria, as a friend and benefactor of many of the rebel factions opposed to President Bashar al-Assad.

But the 18-month-old Saudi-led air campaign in Yemen is under intense scrutiny as the civilian toll mounts and victory against Shiite Huthi rebels proves elusive.

And critics argue that Washington's loyalty to Riyadh diminishes its authority when it tackles Assad and his Russian ally about the civilian toll of the war in Syria.

Philippe Bolopion, deputy director for global advocacy at Human Rights Watch, said the White House warning following the funeral strike was "encouraging."

"It seems like this strike was the straw that broke the camel's back and they finally woke up to the reality of the war in Yemen," he told AFP.

The strike came, he noted, just as "the US, UK and France were rightly condemning the Russian and Syrian war and the indiscriminate bombing of civilians in Aleppo."

"I think they just realized that it was becoming unsustainable," he added.

Washington has criticized Saudi strikes in the past but supports the kingdom's right to investigate its own alleged errors and to pursue its war against the Huthi rebels.

And, while it is concerned, Washington will not be in a rush to cut military ties completely with such a key ally in so many other diplomatic and military efforts in the region.

Nevertheless, on Sunday, Secretary of State John Kerry called Saudi defense minister Deputy Crown Prince Mohammad Bin Salman to express "deep concern."

Simon Henderson, director of the Gulf and energy program at the Washington Institute, said: "They are furious with the Saudis for being indiscriminate in their bombing."

After previous strikes on Yemeni schools and clinics, Saudi officials have accused the Huthis of placing military headquarters in or alongside civilian targets.

- Missile strikes -

"That response sort of works until this attack on the funeral," Henderson told AFP.

Saturday's strike, he said, "looks, frankly, like being a deliberate attempt by Saudi Arabia to kill as many senior Huthis as they could."

According to the State Department, Prince Salman told Kerry the kingdom could envisage a 72-hour ceasefire provided the Huthis agree.

But, and Henderson noted, "Washington has been advising the Saudis to wind this down for months, and has already reduced the level of cooperation in the air operations."

And there seems little immediate prospect of the rebels agreeing.

Former president Ali Abdullah Saleh has allied his security forces with the Huthis to oppose the Saudi-backed return of his successor Abedrabbo Mansour Hadi.

Saleh called for a mobilization to win "revenge" and on Monday the US Navy complained that two missiles had been fired at a US warship from rebel territory.

This aggression and a US interest in maintaining ties already strained by Saudi anger over the Iran nuclear deal explain why Washington has not been more forceful.

David Weinberg, a senior fellow at the Foundation for Defense of Democracies, urged Washington not to bow to "all or nothing" calls to cut military ties.

- Death to America -

He noted that the Huthis had overthrown Hadi's legitimate government and have used the slogan: "Death to America, death to Israel, curse the Jews, victory to Islam."

"The United States would be harmed in my opinion by withdrawing all support precipitately from the Saudi mission in Yemen," he told AFP.

"That having been said," he said, the White House statement "highlights that at least the way in which Saudis are waging war in Yemen is not in line with US interests either."

Somehow, he argued, Washington must find a way to help the Saudis defeat the Huthi threat while minimizing a civilian toll that feeds an anti-American feeling.