FRANKFURT -- Two men armed with a knife and a pistol were stopped trying to board a German plane bound for Tel Aviv last week, but escaped by saying they were undercover security agents, an airline official said on Friday.

German national airline Lufthansa said one of its security officials said he had caught the men trying to get on the plane as it was being prepared for departure to Israel at Frankfurt Airport last Saturday.

"We believe our employee and are taking the matter very seriously," Reuters quoted Lufthansa spokesman Thomas Jachnow as saying.

There was some confusion over whether the men had been carrying out a test to check security levels at Frankfurt Airport, with a spokesman for the Interior Ministry telling a regular government news conference on Friday the German border police had not been involved in such an operation.

A spokesman for the Transport Ministry told the news conference he believed the incident had been a routine test, although he said he did not know whether the men were airport police officers or some other security officials.