LONDON/ SANAA -  The United States and Britain called on Sunday for an immediate and unconditional ceasefire in Yemen to end violence between Iran-backed Houthis and the government, which is supported by Gulf states.

A Saudi-led campaign in Yemen has come under heavy criticism since an air strike a week ago on a funeral gathering in the Yemeni capital Sanaa that killed 140 people according to a United Nations' estimate and 82 according to the Houthis.

On Saturday, a US admiral said a destroyer had again been targeted in the Red Sea in an apparent failed missile attack launched from the coast of Yemen.

US Secretary of State John Kerry said if Yemen's opposing sides accepted the ceasefire then the special envoy to the U.N., Ismail Ould Cheikh Ahmed, would work through the details and announce when and how it would take effect.

"This is the time to implement a ceasefire unconditionally and then move to the negotiating table," Kerry told reporters.

"We cannot emphasise enough today the urgency of ending the violence in Yemen," he said after meeting British foreign minister, Boris Johnson, and other officials in London.

Kerry said they were calling for the implementation of the ceasefire "as rapidly as possible, meaning Monday, Tuesday".

Johnson said the conflict in Yemen was "causing increasing international concern; the fatalities that we're seeing there are unacceptable". "There should be a ceasefire and the U.N. should lead the way in calling for that ceasefire."

Their call came after meetings in London with Saudi Foreign Minister Adel al-Jubeir and senior UAE officials.  Kerry met Iranian Foreign Minister Javad Zarif on Saturday in Switzerland on the sidelines of Syria talks.

"It is a crisis now of enormous proportions with an increasing economic, increasing humanitarian and health crisis, and obviously the military components are troubling to everybody," Kerry said.

Rebels demand int’l probe

Yemen's Shiite Huthi rebels Sunday demanded an international probe into an air strike that killed more than 140 people at a funeral, after a Saudi-led Arab coalition admitted "wrongly" hitting it.

The October 8 raid, condemned by Human Rights Watch as an "apparent war crime", was one of the deadliest since the pro-government coalition launched an air campaign against the rebels and their allies in March 2015.

The coalition's acknowledgement that it wrongly hit the funeral "does not clear its leadership of violating international humanitarian law and all humanitarian norms and conventions", said the rebel-controlled foreign ministry.

UN chief Ban Ki-moon "should form an independent and international investigation committee headed by a high-profile, neutral and international personality as soon as possible to probe war crimes committed by the coalition in Yemen", it said in a statement.

The Riyadh-based coalition acknowledged on Saturday that the air strike in which more than 525 people were also wounded was based on "incorrect information".

It pledged "appropriate action" against those responsible and compensation for families of the victims.

The air strike prompted an international outcry and strong criticism, including from Saudi Arabia's closest Western allies.

Yemen's conflict has killed nearly 6,900 people, more than half of them civilians, since the coalition launched its operations, according to the United Nations.