Four U.S. Army soldiers who went missing while training in Lithuania have died, NATO Secretary General Mark Rutte said Wednesday.

Rutte said he received word of the deaths of the four soldiers while traveling to Warsaw 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.

Few details surrounding the incident were immediately provided by U.S. officials. A U.S. official would say only that the four soldiers were involved in a training accident, The Associated Press reported. The official, who spoke on condition of anonymity, would not comment on the status of the soldiers.

Lithuania, a member of NATO, has often had tense relations with Russia, a key ally of Belarus, since declaring independence from the Soviet Union in 1990. Latvia and Estonia, the other Baltic countries that broke away from the Soviet Union, have had similarly chilly ties with Russia.

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.

The Associated Press contributed to this report.