German police arrested a man on Wednesday after two U.S. airmen were shot dead and two wounded in an incident on a U.S. Army bus at Frankfurt airport, authorities said.

Police seized and arrested the alleged gunman in the terminal building, reportedly still with large amounts of ammunition and his handgun.

Spiegel magazine's online edition cited investigators as saying preliminary enquiries suggested this was a planned attack, and not the result of an argument, as had first been suggested.

According to the magazine, the man entered the bus full air force personnel, who had just arrived in Frankfurt from Britain, and opened fire.

The 21-year-old Albanian from Kosovo cried "Allahu Akhbar" ("God is Greatest"), reports quoted witnesses as saying, before opening fire in the bus outside one of Europe's busiest airports.

"Whether the incident was linked to terrorism I cannot say at this stage," Boris Rhein, interior minister for Hesse state told reporters.