U.N. spokeswoman Hua Jiang said the helicopters were fired on as they took off from Abyei town late Tuesday. She said about 14 rounds were fired from positions close to the U.N. compound. No helicopters were hit.

Gunmen from an Arab tribe fired on four U.N. helicopters taking off from a disputed north—south Sudan border town, an official said on Wednesday, while Sudan’s president gave northern troops a “green light” to attack southern forces if provoked.

U.N. spokeswoman Hua Jiang said the helicopters were fired on as they took off from Abyei town late Tuesday. She said about 14 rounds were fired from positions close to the U.N. compound. No helicopters were hit.

Both Sudan’s north and south claim Abyei, a fertile region about the size of Connecticut that is located near several oil fields. Northern tanks and soldiers rolled into the disputed region Saturday following an attack on a northern army convoy last week.

Southern Sudan voted in January to secede from the north on July 9, but the north’s occupation of Abyei has greatly strained north—south relations. The two regions fought a two—decade—plus civil war that claimed 2 million lives, and fears are rising Abyei could send the sides back to war.

Sudanese President Omar al—Bashir gave his army a green light to attack southern forces without seeking permission from Khartoum if they feel provoked, according to a statement carried Wednesday by Sudan’s state news agency.

Since an attack last Thursday by southern troops on northern and U.N. forces, northern troops have moved south into Abyei, sending thousands of civilians fleeing. Northern aircraft are reported to have made bombing runs in the region, and the U.N. said that gunmen set homes ablaze and looted in Abyei town.

The south has called the move into Abyei an act of war but has not yet responded with force.

The south’s secession vote was promised in a 2005 peace deal that ended the north—south civil war. The conflict over Abyei could scuttle the Comprehensive Peace Agreement that promised the January independence referendum and the July 9 independence date.

A referendum on Abyei’s future was supposed to have been held simultaneously, but the two sides could not agree on who was eligible to vote, and Abyei’s referendum wasn’t held. The black African tribe of the Ngok Dinka, which is allied with the south, and the Arab tribe of Misseriya, which is allied with the north, both claim the area.

Ms. Jiang said that Misseriya tribesmen are moving south into Abyei town, though she did not know how many.

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