An international non- governmental organization monitoring Sudanese air space on Wednesday said satellite photos showed evidence of war crimes committed at Sudan's disputed oil-rich area of Abyei, which is controlled by Sudanese Armed Forces (SAF).

"These images provide supporting documentary evidence of war crimes against humanity in Abyei," said John Bradshow, Head of Sentinel Project Group for monitoring Sudanese villages via satellite, in a statement.

The statement added that "build-up of tanks, helicopters, infrastructure improvements, troop strength and deployment of forces along main roads within striking range of Abyei, indicate that the invasion of Abyei was premeditated and well-planned."

The SAF has recently taken control of Abyei in response to an attack by the Sudan People's Liberation Army (SPLA) against an SAF convoy which resulted in the deaths of 22 of its soldiers.

The United Nations Mission in Sudan (UNMIS) on Wednesday said four of its helicopters were fired at as they were taking off from Abyei.

In a statement Wednesday, the mission said that "about 14 rounds were fired from positions close to the U.N. compound," adding that all helicopters landed safely.

As the UN and many big countries asked the Sudanese army to pull out of Abyei, the Sudanese President Omar al-Bashir on Tuesday said that "Abyei belongs to north Sudan" and that the Sudanese army would not pull out of it.

"The Sudan People's Liberation Movement (SPLM) wants to make Dinka tribesmen the first class citizens and Mesiria tribesmen second class citizens in Abyei and we will never accept that," al- Bashir said.

He reiterated that Abyei belongs to north Sudan, saying "the army will not withdraw from Abyei. We do not want the U.S. carrot and we are not afraid of its stick."

Al-Bashir further lashed out at the SPLM which he said has gone too far in its provocations against the Sudanese army, adding that the movement wanted to impose a reality in Abyei via mobilizing its troops in the area and recoiling from fulfilling its repeated promises to pull its troops out of Abyei.

"The SPLM thought the Sudanese army was unable to fight the war as it was occupied by the situation in Darfur, but we are ready for war," said al-Bashir.

A referendum on Abyei was supposed to be held in Jan. 9, 2011 coincident with the south Sudan referendum, but it was postponed in the wake of a difference between the NCP and the SPLM over who has the right to vote in the referendum. The NCP said the Mesiria tribe has the right to vote, while the SPLM insisted that only Dinka Nkok tribe has that right. Both tribes claim the right over Abyei.