Four U.S. Army soldiers were killed during a training exercise in Lithuania, NATO Secretary General Mark Rutte said Wednesday, speaking with reporters in Poland. The U.S. embassy in Vilnius said earlier in a statement posted on social media that the four soldiers had gone missing during training, and it thanked the Baltic nation’s military and police for helping American forces search for them. 

A U.S. official would say only that the four soldiers were involved in a training accident.

“The news came out about four American soldiers who were killed in an incident in Lithuania,” Rutte told reporters in Warsaw. “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.” 

The U.S. embassy said in its social media post that the troops went missing from a training area near Pabrade, near Lithuania’s far eastern border with Belarus, a nation closely allied with Russia. The embassy did not say when the soldiers went missing.

“The soldiers, all from 1st Brigade, 3rd Infantry Division, were conducting scheduled tactical training at the time of the incident,” the embassy’s statement said. 

Lithuanian news outlet Delfi said the soldiers went missing along with a tracked vehicle, which typically refers to a tank or armored personnel carrier.