An Australian army helicopter ditched into the waters off Australia’s northeast coast during a nighttime training mission, leaving four crew members missing and prompting military officials to pause a broader, large-scale multinational defense exercise on Saturday. Ditching refers to a difficult emergency landing on water.

U.S. and Australian personnel have been conducting a search but have not yet found the missing crew, Australian Deputy Prime Minister Richard Marles said Saturday.

“I know I speak on behalf of all four of us when I say that our thoughts and prayers are very much with the missing aircrew and their families,” Marles told reporters at a news conference in Brisbane, Australia, standing next to Australian Foreign Minister Penny Wong and U.S. Secretary of State Antony Blinken and Secretary of Defense Lloyd Austin, who pledged any assistance needed to find the missing crew.

The MRH-90 Taipan helicopter ditched near Hamilton Island in Queensland just after 10:30 p.m. local time on Friday, Australian officials said. Search efforts began immediately thanks to another helicopter that had been flying next to the downed chopper. Australian officials said they have paused the Talisman Saber exercise, a multinational drill that began last week.

Although the exercise is primarily a bilateral drill between Australian and American troops, it involves more than 30,000 troops from 13 countries, including those from several Asian partners and NATO member states. The biennial drill is the largest combined training involving Australia and the United States.

“The United States has no closer or more reliable ally than Australia,” Blinken told reporters.

Cooperation between the countries expanded two years ago, when they and Britain made a deal, called AUKUS, to deepen their partnership on security issues. One key part of the deal was for Australia to acquire nuclear submarines using U.S. technology, a plan that would help the United States expand its power projection into the Pacific as worries grow about China.

The leaders announced a slew of investments and defense cooperation in the air, sea, land and in space, all aimed at bolstering the security relationship between the two countries, which they noted had fought side-by-side in every war for more than a century. Now the joint threats are focused on Russia and China, and they said that the Australian-U. S. relationship has a key role to play in addressing the challenge.

The MRH-90 helicopter has been the subject of controversy. Although Australia’s military bought almost four dozen of them for the army and navy, the Defense Ministry listed the acquisition as a “project of concern” in 2011. Multiple equipment malfunctions and crashes, including one in March, have compelled officials to ground the fleet several times.