U.N. officials are warning of a humanitarian crisis as U.K. airstrikes on Yemen continue.

The U. N. human rights chief has warned of a “worldwide humanitarian crisis” as a Saudi-led coalition attacks Yemeni civilians.

U.S.-led coalition forces in Yemen began an offensive Monday targeting forces loyal to Yemen’s internationally recognized government and rebels.

Saudi-led forces launched their latest offensive against the rebels Tuesday, targeting fighters and the Houthis, an alliance of Yemeni fighters allied with Iran.

On Wednesday, the U. S. military also announced it would begin bombing positions of the Houthi-allied Houthi rebels in the north, including in the capital, Sanaa.

A U. K. airstrike in Yemen on Monday killed at least 12 civilians and injured dozens more, according to the U K. military.

Earlier in the day, the coalition announced it was launching a military campaign to retake the northern city of Taiz, home to a large Al Qaeda stronghold and where the U U. s. military said it would target military installations and a military camp in the city.

In a separate statement, U.n.

Secretary-General Antonio Guterres called for a swift, independent investigation into civilian deaths and the impact of coalition airstrikes.

“There can be no justification for the bombing of civilians,” he said.

He added that a thorough, independent inquiry “must be conducted in the context of a clear international mandate to investigate and prosecute such crimes.”

Uyghurs in Taiz have long sought independence from the central government and are seeking to form their own nation, but the Saudi-backed coalition has been carrying out airstrikes in support of the Shiite rebels.

U.k.

Prime Minister David Cameron said Wednesday that “there are serious concerns” about civilian casualties from the Saudi air strikes.

More from the Washington Post: Uygurs in Yemen accuse Saudi-aligned coalition of targeting civilians and rebels U. Saudi Arabia’s warplanes carried out airstrikes Monday on the Houtha-aligned Houthi group in Yemen’s north, killing at least 13 people and wounding dozens more in a massive escalation of a conflict that has killed more than 10,000 people and displaced more than 20 million people.

U.

Saudi-allies have been targeting Al Qaeda-linked Houthi fighters in Yemen, killing hundreds of civilians and forcing millions to flee.