The United Nations says it is investigating reports that U.N. peacekeepers in Sudan stayed in their barracks during recent fighting between forces from the north and south.

A spokesman for the U.N. peacekeeping department says the agency has sent its top military advisor to Sudan's disputed Abyei region to assess the performance of U.N. troops deployed there.

Several U.N. diplomats are criticizing the U.N. peacekeepers from Zambia for failing to carry out their mandate, which includes protecting civilians in Abyei.

North Sudanese forces seized control of the contested, oil-rich Abyei region in May. Sudan has rejected calls from the United States, United Nations and south Sudan to remove its troops. The fighting has caused tens of thousands of people to flee the region and raised fears of renewed civil war only weeks before south Sudan secedes from the north on July 9.

The U.N. Security Council Friday strongly condemned the takeover of Abyei. It called on Sudanese forces to immediately ensure an end to all looting, burning and illegal resettlement in the region.

The council described Sudan's military operations in Abyei a “serious violation” of the 2005 peace agreement between north and south Sudan.

The two sides fought a 21-year civil war that ended in 2005.

South Sudan is set to declare independence next month after voting to split from the north in a January referendum. Abyei was scheduled to decide at the same time on whether to join the north or the south, but that referendum failed to happen because the sides could not agree on who was eligible to vote.

Sudan's government in Khartoum recently informed the United Nations that it wants the U.N. peacekeeping force on its southern border to leave its territory when south Sudan becomes independent.