A group of men armed with bats and iron bars attacked a center for political asylum-seekers in southern Norway, leaving more than 20 people injured, the facility's director said Friday.

Police were investigating the violence late Thursday at the Nordbybraaten center, about 40 miles south of Oslo, but said they have not arrested anyone.

Center director Ole Morten Lyng said between 40 and 50 men were involved in the "planned" attack. He did not know how serious the injuries were.

Norwegian news agency NTB quoted Lyng as saying the attackers were Chechens, but he later told The Associated Press that their nationalities were not clear.

NTB said the center previously has had conflicts between groups of asylum-seekers. The facility houses some 200 people from Iraq, Afghanistan, Iran, Russia, Somalia and Eritrea.

It was the second attack in a week at an asylum center in Norway. On July 18, police said a 16-year-old Somali boy was seriously wounded after several shots were fired at an accommodation center for asylum-seekers in Asker, outside Oslo.

No suspects have been arrested in the shooting, and it wasn't immediately clear whether the cases were related.