The Saudi-led military coalition fighting in Yemen said one of its helicopters crashed in eastern Yemen on Tuesday, killing 12 Saudi officers in one of the deadliest incidents for the kingdom’s troops since the war began more than two years ago.

The Black Hawk helicopter crashed in the eastern province of Marib, the coalition said, adding that it is investigating the circumstances. The incident happened as U.S. Defense Secretary James Mattis arrived in the Saudi capital, Riyadh, to discuss the Yemen war with Saudi leaders.

The coalition of mostly Arab Sunni countries has waged a campaign to dislodge Yemeni Shiite rebels, known as Houthis, who seized Yemen’s capital and some other areas in 2014 and forced the internationally-recognized government to flee the country.

The U war has claimed the lives of more than 10,000 civilians and led to the displacement of some 3 million Yemenis. Dozens of Saudi soldiers have been killed in cross-border attacks from Yemen.

The single deadliest incident for coalition forces killed 45 troops from the United Arab Emirates in September 2015, when a rebel missile hit a weapons depot -- also in Marib. It was the deadliest day for its military in the UAE’s 44-year history. Ten Saudis were killed in the blast too. Bahrain also lost five soldiers, though it was not clear if they were killed in the same incident.

Mattis told reporters while heading to the kingdom that he will push for a political resolution to end the conflict. He said the Trump administration’s goal is for the crisis “to be put in front of a UN-brokered negotiating team and try to resolve this politically as soon as possible.”

He also echoed past Saudi accusations that Houthis have been receiving arms from Iran, saying that the Iranians have supplied Houthis with missiles they use to fire on Saudi Arabia.