Sudan's president gave northern troops a "green light" to attack southern forces if provoked, while gunmen from an Arab tribe fired on four U.N. helicopters taking off from a disputed border town at the heart of a new north-south conflict, officials said Wednesday.

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 the attack on a northern army convoy Thursday, raising fears the dispute could trigger a return to civil war in Africa's largest nation.

President Omar al-Bashir said his troops do not need permission from Khartoum to attack southern forces if they feel provoked, the state news agency SUNA said. He accused the U.S. of double standards because he said it protested loudly over the occupation of Abyei by the north, but less loudly over the preceding attack on northern troops and U.N. forces.

President Barack Obama, speaking at a news conference in London, called for the rapid reinforcement of U.N. peacekeeping troops in the Abyei region, from which tens of thousands of civilians have fled over the last week.

Some U.N. peacekeepers remain in Abyei, although U.N. spokeswoman Hua Jiang said U.N. helicopters were fired on as they took off from there late Tuesday. She said about 14 rounds were fired from positions close to the U.N. compound. No helicopters were hit.

Southern Sudan voted in January to secede from the north, and it is scheduled to declare independence in July. 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.

Northern aircraft are reported to have made bombing runs in the region, and the U.N. said gunmen set homes ablaze and looted in Abyei town. The accusations were supported by satellite images released Wednesday by the Satellite Sentinel Project, which showed burnt structures north of Abyei town and fires burning in the region.

"These images provide supporting documentary evidence of war crimes and crimes against humanity in Abyei," said Enough Project Executive Director John C. Bradshaw. "It is imperative that the international community not reward these crimes by allowing the government of Sudan to improve its bargaining position at the negotiating table."

The group also released pictures of attack planes and Antonov transport craft, which the Sudanese government use as bombers, at an air base within striking distance of Abyei. Charlie Clements, the Harvard Carr Center Executive Director, said the military buildup indicates that the invasion of Abyei was premeditated and well-planned.

The south has called the move into Abyei an act of war but has not yet responded with force. Its army is far weaker than the north's and it fears endangering its upcoming independence.

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.

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

The U.S. has said it would reward al-Bashir's government for a successful southern independence process by removing Sudan from the list of state sponsors of terror, helping it get relief for its debt burden and normalizing relations with the U.S. Princeton Lyman, Obama's Special Envoy to Sudan, said this week that those rewards are in jeopardy if the independence process is not completed.

But al-Bashir indicated he was no longer interested in those items.

"We no longer fear the American stick nor do we desire its carrots," Sudan's news agency quoted him as saying.

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Associated Press reporter Mohamed Osman in Khartoum, Sudan contributed to this report.