27 soldiers from the United Arab Emirates and Bahrain were killed on Friday while taking part in a Saudi-led military campaign against Yemen’s dominant Houthi group, in the deadliest attack on the Gulf forces since the offensive began.

Saudi Arabia and a coalition of other Sunni Muslim Gulf states have been fighting since March to restore Yemen’s exiled government and repel the Iran-allied Shi’ite Houthis, who took control of the capital Sanaa in September last year. UAE state news agency WAM said on Friday that 22 Emirati soldiers were killed in Yemen, while Bahrain’s official news agency BNA said five soldiers were killed while on duty protecting the southern borders of neighbouring Saudi Arabia.

“A rocket and an explosion at a weapons cache has targeted the martyrs,” Anwar Gargash, UAE minister of state for foreign affairs, said on Twitter on Friday. The Houthis said they fired a rocket at a weapons cache in a camp used by Gulf coalition forces in the central Marib area, killing dozens of Emirati and Yemeni soldiers and destroying a number of Apache helicopters and armed vehicles. Residents in Marib said they saw fire raging at the camp and plumes of smoke.

Before the latest incident, at least five Emirati soldiers had been killed in Yemen since the offensive began. Militias and army units loyal to President Abd-Rabbu Mansour Hadi, who is taking refuge in Saudi Arabia, have made advances toward the Houthi-controlled capital in the last two months. But the group remains ensconced in Yemen’s north, and military and civilian casualties mount in nationwide combat every day.

The coalition has been supporting anti-Houthi fighters with air strikes, military training and the delivery of tanks and heavy artillery. Gulf states regard the Houthis as a proxy of their arch-rival, Shi’ite Iran, while the Houthis say they are fighting a revolution against corrupt officials beholden to the West.