WARSAW, Poland (AP) — 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.