NATO Secretary General Mark Rutte said Wednesday that four U.S. soldiers who went missing while training in Lithuania have died, but that he did not yet know the details.

A U.S. official would say only that the four soldiers were involved in a training accident. The official, who spoke on condition of anonymity, would not comment on the status of the soldiers.

Rutte said during a trip to Warsaw that he had received word of the deaths of the four soldiers and that his thoughts were with their families and with the United States.

“This is still early news so we do not know the details. This is really terrible news and our thoughts are with the families and loved ones,” Rutte told reporters in Warsaw.

A statement from U.S. Army Europe and Africa public affairs in Wiesbaden, Germany said the soldiers were conducting scheduled tactical training at the time.

Lithuanian public broadcaster LRT reported that four U.S. soldiers and vehicle were reported missing Tuesday afternoon during an exercise at the General Silvestras Žukauskas training ground in Pabradė, a town located less than 10 kilometers (6 miles) from the border with Belarus.

The Baltic countries of Lithuania, Latvia and Estonia are all NATO members and have often had chilly ties with Russia, a key ally of Belarus, since declaring independence from the Soviet Union in 1990.

Relations soured further over Russia’s full-scale invasion of Ukraine in 2022, and Lithuanian President Gitanas Nausėda has been one of the most outspoken supporters of Ukraine in its fight against Russian President Vladimir Putin’s forces.