Khartoum, Sudan (CNN) -- The United Nations Mission in Sudan on Monday condemned "the burning and looting currently taking place by armed elements in Abyei," a disputed oil-rich border district, and called on the government to put a stop to it.
"We are not sure yet who is responsible," mission spokesman Kouider Zerrouk told CNN. "We have no word on casualties yet, but we are calling the government of Sudan to take responsibility, as Abyei is under control of the Sudanese Armed Forces."
The humanitarian agency Medicines Sans Frontieres, also known as Doctors Without Borders, suspended some of its services in Abyei and issued a statement reading: "Nearly the entire population of Abyei town fled the city, which is now almost empty. MSF teams have suspended primary health care activities in Abyei and have now reached the fleeing population."
Fighting in the district of Abyei broke out May 19 when Southern Sudanese forces attacked a U.N. Mission in Sudan convoy escorting northern Sudanese Armed Forces.
The attack, which resulted in the deaths of at least 22 soldiers, was followed by retaliation by the Sudanese army, which took control of the town and expelled southern Sudanese units.
The U.N. compound was shelled by mortar rounds, wounding two peacekeepers, said a statement from U.N. Secretary-General Ban Ki-moon's press office.
The Sudanese armed forces said the military in Southern Sudan was behind the May 19 attack, but the Sudan People's Liberation Army denied responsibility.
Sudanese President Omar al-Bashir issued a decree Saturday dissolving Abyei's administration. The area had been governed by representatives from the north and south.
"The secretary-general remains deeply concerned for the safety of the civilian population of the area, the vast majority of whom (have) been forcibly displaced due to the fighting," the U.N. statement said Sunday. "The secretary-general calls on both parties to immediately cease their military operations, withdraw all forces and armed elements from Abyei and desist from further acts of antagonism."
Members of a U.N. Security Council delegation who were visiting Khartoum called on northern and southern Sudanese forces to "refrain from the use of force and not engage in unilateral action" as tension rises in the disputed area.
In a statement read Sunday to journalists by the French ambassador to the Security Council, the council denounced both sides.
"The members of the Security Council condemn the escalatory military operation being undertaken by the Sudanese Armed Forces, which have taken control of the area in and around Abyei town," the statement read.
It also condemned "the attack by Southern forces against the UNMIS convoy escorting Sudanese Armed Forces ... in Abyei."
"This is a worrying crisis that has evolved," said U.S. Ambassador Susan Rice. "We are watching very carefully and will take the appropriate steps as the situation unfolds."
Earlier, Sudanese government representative Amin Hassan Omar described the southern Sudanese attack on the U.N. and Sudanese forces as "illegal."
"Since December there has been an occupying force from the Sudanese People Liberation Army under the name of the South Sudan Police Force in contradiction to the Comprehensive Peace Agreement," he said.
"The Sudanese Army tried to cleanse the area of a force that sought to enforce the position of one party as a matter of reality when negotiations are still taking place to find a solution to this complicated problem," Omar added.
The peace agreement signed in 2005 between the government of Sudan and southern Sudanese rebels ended decades of war between the predominantly Arab and Muslim north and predominantly black animist and Christian south.
South Sudan is set to become an independent state in July after an overwhelming majority of southern Sudanese voted last January to secede. Unresolved issues include the fate of Abyei.
Col. Philip Aguer, spokesman for the Sudan People's Liberation Army, the military wing of the Southern Sudanese government, described events in Abyei as a "coup against the peace agreement."
"Abyei has been attacked from air and ground. ... We are calling upon the international community to protect the civilian population," he said.
Medicines Sans Frontieres said many of the evacuees had fled 40 kilometers (24 miles) south to Agok. They include 42 people wounded over the weekend who are being treated at the aid group's hospital there.
The Commission of the African Union responded Sunday to the developments in Abyei "with very grave concern," according to a statement.
"The AU Commission would like to urge all the parties to immediately withdraw all the unauthorized forces from the area ... and to restrain from any provocative actions," the statement read. "The AU Commission calls on the parties to resume the dialogue towards a political settlement on Abyei by returning to the negotiating table."
Journalist Ismail Kushkush contributed to this report.