A gunman shot dead two U.S. soldiers and injured two people seriously in a U.S. Army bus at Frankfurt airport Wednesday, said Boris Rhein, interior minister for the state of Hesse.

The gunman was apparently a Kosovo national, he said.

Security round the airport has been tightened and an investigation is under way, Rhein said.

Whether the incident was linked to terrorism I cannot say at this stage, he told journalists.

A spokesman for Frankfurt airport operator Fraport said the shooting took place in a U.S. Army bus in front of Terminal 2.

Chancellor Angela Merkel, speaking in Berlin, confirmed the deaths.

We don't know the details but I would like to express how upset I am. We have to do everything we can to find out what happened, she told a joint news conference with Portuguese Prime Minister Jose Socrates.

The U.S. Army and Air Force and German police at the airport were not immediately available for comment