UNITED NATIONS - The United States is leading international warnings to Sudan and South Sudan to step up efforts to reach a peace accord this week or face possible UN sanctions.

The UN Security Council has given the rival neighbors, who this year came close to all-out war, until Thursday to make their peace.

Tentative talks are being held in Addis Ababa. Diplomats said however that while no accord is expected the 15-nation council will probably hold back from ordering immediate action.

"It appears increasingly unlikely that a comprehensive agreement on outstanding issues will be reached" by the August 2 deadline, said Susan Rice, the US ambassador to the United Nations.

South Sudan broke away from Sudan in July last year but no deal has ever been made to set their frontier, how to share revenues from oil reserves that straddle the border, or how to settle citizenship disputes.

"The United States calls on the parties to fulfill immediately their obligations under Resolution 2046," Rice said in a statement referring to the resolution passed by the Security Council in May which set the deadline.

"The US wishes to reiterate the UN Security Council's decision -- in the event that any or all of the parties have not complied with the decisions set forth in this resolution to take appropriate additional measures under Article 41 of the (UN) Charter as necessary," the envoy added.

The United States has taken a tough line with Sudan, which is accused of staging an air raid across the border on July 20. Other Security Council nations say that pressure has to be put on both sides.

"The United States strongly condemns Sudan's July 20 bombing" in South Sudan's Bahr El Ghazal state, said Rice.

"This incident constitutes a serious violation of Resolution 2046 and marks a recurrence of violence, which had abated in the period since the resolution's adoption."

The UN resolution ordered the two countries to halt hostilities, withdraw their forces from the disputed Abyei frontier region and make a comprehensive accord in three months.

Sudan has however kept a special police force to protect oil facilities in Abyei and has yet to agree a demilitarized border zone map. It also faces international criticism over its war with rebels in South Kordofan and Blue Nile states, on the border with South Sudan.

Tens of thousands of refugees from the conflict have crossed into South Sudan and Ethiopia.

Rice highlighted international concern over what the United Nations has called a humanitarian crisis in South Kordofan and Blue Nile, where the Khartoum government has severely restricted access to aid agencies.

Sudan accuses the South of supplying the Sudan People's Liberation Movement-North (SPLM-N), which is battling government forces.

The United States praised South Sudan for keeping to most provisions of the UN resolution but Rice said: "At the same time, the United States reiterates the Security Council's call to cease support to rebel groups."

Former South African president Thabo Mbeki is leading African Union efforts to mediate between the two rivals at the Addis Ababa talks, which started last week, but diplomats have cited no progress.

Mbeki is expected to brief the Security Council on August 9, when first consideration of potential sanctions is expected to be discussed.

The Security Council is expected to increase pressure on Sudan this week with a resolution renewing the UN-African Union peacekeeping mission in the conflict-stricken Darfur region.

Sudan is angry because the United Nations wants the mission to be mandated to investigate whether the Lord's Resistance Army is operating in South Darfur. Sudan's UN envoy has warned that his country could withdraw cooperation if the LRA measure is passed.