|The Sudanese northern army accuse the south of carrying out the attacks in the disputed Abyei region [Reuters]
The United Nations has called for an "immediate cessation of hostilities" in Abyei following days of clashes, including artillery fire, in the disputed Sudanese border region.
The armies of North and South Sudan, which is due to claim independence from Khartoum later this year, have accused each other of launching attacks in the region.
"We strongly encourage all parties to resume dialogue towards reaching a lasting political settlement," the United Nations Mission in Sudan (UNMIS) said in a statement on Saturday.
"The United Nations Mission in Sudan is deeply concerned over the troop build-up and reported fighting, including the use of heavy artillery and bombing in the Abyei area," it continued.
A spokesman for the southern army, the Sudan People's Liberation Army (SPLA), told the AFP news agency that fighting had intensified on Saturday and accused northern forces of conducting bombing raids and ground attacks.
"We fear that SAF [the northern army] are trying to advance to Abyei town," he said. "Things are bad but we don't have full details at present. It has not been a good day."
Artillery fire erupted hours after the north accused the south of ambushing a convoy in the border area, the United Nations said.
"We heard artillery exchanges in Abyei in Todach and Tagalei but we don't know who is fighting whom," said Kouider Zerrouk, spokesperson for the UN Mission in Sudan (UNMIS) on Friday.
The north accused the SPLA of attacking a convoy of Sudanese soldiers and UN peacekeepers late on Thursday in Dokura north of Abyei town.
"Abyei is now a war zone," said Sadiq Amer, deputy head of northern intelligence and security forces, adding that at least 22 soldiers had been killed in the attack.
"The troops were ambushed without any warning," he told reporters in the capital Khartoum.
"We know that they are mobilising huge numbers of troops, not only in Abyei but along the 1956 border line," he said. "They should know that the Sudanese armed forces are ready."
The SPLA denied responsibility for the attack, which the United Nations said had taken place on a convoy of northern troops escorted by UN peacekeepers, under a deal for both sides to withdraw forces from the disputed territory.
The United States, one of the main backers of Sudan's landmark 2005 peace deal, deplored the attack and urged both sides to stop all unauthorised military actions in Abyei.
"Political leaders on both sides must take responsibility now to ensure that this situation does does not escalate into a wider crisis," State Department spokesman Mark Toner said.
South Sudan voted to become independent in a referendum in January. It agreed to a peace deal with the north in 2005, but tensions are high in oil-producing Abyei, where both sides have built up forces.
The north has said it will not recognise the state unless the south gives up its claim to Abyei.