German authorities say a man armed with a knife has taken four schoolchildren hostage in northwestern Germany.

Officials say the man, described by witnesses as in his forties, boarded a bus filled with children returning home from school Tuesday. He ordered about 10 of the children to get off, and then took four of them into a house in the town of Ennepetal, near Cologne, where he is holding them hostage. Earlier reports had put the number of hostages at seven.

Police have surrounded the house.

German television networks identify the man as being of Iranian origin and say he is demanding that his family be allowed to leave Iran and travel to Germany.

Some information for this report provided by Reuters and AP.