ENNEPETAL, Germany - A man armed with a knife forced at least four children off a bus in northwest Germany today and held them hostage in a nearby house, police said.

The man forced between four and seven children aged 10 and 11 from a public bus at about 1 p.m., police spokeswoman Sandra Zwick said.

Police surrounded the house on the edge of the town, which is located about 40 miles northwest of Cologne. The man's motives were not immediately known, police said.

The man, described by witnesses as being in his 40s, took about 10 children off the bus but let several go on the street. He forced the others toward a nearby house where a woman was returning home and opening her front door, police said.

The kidnapper pushed the woman aside, shoved the children into the house and locked the door. Police said they were trying to establish contact with the man.

The children were on a public bus packed with children on their way home from school, police said. Initially, police said the children were on a school bus.