LONDON (Reuters) - 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 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 U.S. admiral said a destroyer had again been targeted in the Red Sea in an apparent failed missile attack launched from the coast of Yemen.

U.S. 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 emphasize 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”.

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The UN’s special envoy said he had been in contact with the Houthi’s lead negotiator and the government of President Abd-Rabbu Mansour Hadi. He also said he hoped for “clearer plans” for a ceasefire in the coming days

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.

He added that the release of two American prisoners by Yemen’s Houthi and the evacuation of Yemeni civilians wounded in a Saudi airstrike were “an important humanitarian gesture by the Saudis to address the humanitarian concern”.