DUBAI, United Arab Emirates –  Forty-five members of the United Arab Emirates' military were killed while taking part in Saudi-led operations in Yemen against Shiite rebels known as Houthis, the official news agency WAM said Friday, the largest single loss for the Gulf nation's military to date in the war.

Pro-government Yemeni security officials said the troops were killed Friday when a Houthi missile hit a weapons storage depot near their position in the province of Marib, about 75 miles east of the capital Sanaa. Officials from the Houthi media office in Sanaa confirmed they fired a Soviet-era Tochka missile.

The WAM news agency didn't specify the role of the personnel in Yemen. The seven-state Emirates federation is one of the most prominent members of the Saudi-led coalition, which aims to roll back gains by the Shiite rebels and their allies in the deeply impoverished Arabian Peninsula country.

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.

Friday was by far the deadliest day for the Emirati military since the conflict began, and the deaths are believed to be the country's highest number of military casualties since the UAE federation was founded in 1971. U.S. Secretary of State John Kerry phoned the Emirati foreign minister, Sheikh Abdullah bin Zayed Al Nahyan, soon after the deaths were announced to express his condolences, WAM said.

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.

Fighting in Yemen pits the Houthis and army units loyal to former President Ali Abdullah Saleh against forces loyal President Abed Rabbo Mansour Hadi, who is in self-imposed exile in Saudi, as well as southern separatists and local militias.

The Saudi-led and U.S.-backed coalition has been launching airstrikes against the rebels since March, in an operation that aims to roll back gains by the Shiite rebels and their allies. Houthis have captured more territory in Yemen after taking control of the capital, Sanaa, last September.

Clashes between Houthis and pro-government forces, as well as airstrikes from the Saudi-led coalition, intensified this week in Marib 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 Jawf province, then to Saada, the Houthis' stronghold in the north, the security officials said.

They spoke on condition of anonymity because they weren't authorized to brief reporters.

Emirati ground forces and hardware have been playing an increasingly prominent role in the conflict in recent weeks, though officials have not made clear the full extent of their role or the numbers of troops involved.

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 Qaeda in Yemen and was flown out aboard a UAE military aircraft.

The Gulf nation's involvement in the Yemen conflict follows the introduction of a law last year requiring compulsory military service for adult males.

The U.S.-allied country, which includes Dubai and the oil-rich capital of Abu Dhabi, last month announced the creation of a new national holiday, Martyrs Day, to commemorate those killed in the line of duty.