Forty-five troops from the United Arab Emirates were killed in Yemen while taking part in Saudi-led operations against Houthi rebels, the Gulf nation said Friday, in the deadliest day for its military in its 44-year history.

The UAE's minister of state for foreign affairs, Anwar Gargash, said the troops were killed when a missile struck an ammunition depot. On his official Twitter feed, he said the "cowardly attack will not deter us."

Pro-government Yemeni security officials said the missile strike took place in the province of Marib, about 75 miles east of the capital Sanaa. Officials from the media office of the Houthi movement in the Yemeni capital Sanaa confirmed they fired a Soviet-era Tochka missile in the area. The officials spoke on condition of anonymity because they were not authorized to talk to the press.

The deaths pointed to the increasingly prominent role of the Emirates on the ground in Yemen's war — both in troops and hardware — though the government has never made clear the full extent of their role or the numbers of troops involved.

The UAE's news agency, WAM, initially reported 22 members of the military were killed Friday but later reported that 23 more had died of their wounds. It gave no details on how they were killed or on what their role in the conflict was.

The U.S.-allied Emirates, a federation of seven small Gulf states including Dubai and the oil-rich capital of Abu Dhabi, is one of the most prominent members of the Saudi-led coalition, which aims to roll back gains by the Houthis and their allies in the deeply impoverished Arabian Peninsula country. The Saudi-led and U.S.-backed coalition, made up mainly of Gulf nations, has been launching airstrikes against the Houthis since March. But the UAE is the only country that has acknowledged having troops on the ground in Yemen in the conflict.

The Houthi fighters took over Sanaa a year ago and soon after swept over other parts of the country, driving President Abed Rabbo Mansour Hadi into self-imposed exile in Saudi Arabia. The Houthis are backed by army units loyal to former President Ali Abdullah Saleh, and fighting has raged in multiple parts of the country between those forces and those loyal to Hadi as well as southern separatists and local militias opposed to the Houthis.

Bahrain's state news agency also reported Friday that five of its soldiers were killed while "defending the southern border of Saudi Arabia." It didn't give specifics. Yemen is the only country on Saudi Arabia's southern border where there is fighting, and Houthis have frequently shelled across the frontier.

The Emirati deaths came amid heavy clashes and intensified coalition airstrikes in Marib province, as the opposing sides gear up for a critical battle over the coming days. Pro-government forces want to clear Marib province of Houthi fighters, then proceed on to neighboring Jawf province to the north then to Saada, the Houthis' stronghold in the north, the security officials said.

The toll was the Emirates' highest number of combat casualties since the federation was founded in 1971. Around six of its troops were killed fighting as part of the U.S.-led coalition that drove the Iraqi forces of Saddam Hussein out of Kuwait in 1991. At least five other members of the Emirati military have been killed in Yemen this year, and another died during training exercises related to the operation in Saudi Arabia.

On Friday, U.S. Secretary of State John Kerry phoned the Emirati foreign minister, Sheikh Abdullah bin Zayed Al Nahyan, after the deaths were announced to express his condolences, WAM said.

The Yemen deployment is part of an increasingly assertive military policy by the UAE in the region. Its warplanes are believed to have carried out strikes against armed groups in Libya in coordination with Egypt. The Emirates last month freed a British hostage being held in Yemen in what authorities said was a military intelligence operation. The captive, Robert Douglas Semple, had been kidnapped 18 months earlier by al-Qaida in Yemen and was flown out aboard a UAE military aircraft.

Last year, the Gulf nation introduced a law requiring military service for adult males. It created a new national holiday last month, Martyrs Day, to commemorate those killed in the line of duty.

The Associated Press