The United Nations on Friday accused Sudan of bombing a refugee camp in South Sudan and called for an investigation into the attack, evidence of the escalating tensions between Khartoum and Juba.

Yesterday UNMISS (the U.N. mission in South Sudan) confirmed that the Sudan Armed Forces dropped at least two bombs near the Yida refugee camp ... with unknown casualties, U.N. peacekeeping chief Herve Ladsous told a meeting of the U.N. Security Council.

Khartoum dismissed the allegations.

There have been no bombings, Sudanese U.N. ambassador Daffa-Alla Elhag Ali Osman told the council.

These are fabrications, he said afterwards to reporters, adding that the reports of the air strike had been concocted by media agencies supporting anti-Khartoum rebels.

U.S. Ambassador Susan Rice reiterated Washington's condemnation of the attack, calling it outrageous. She added that Osman had blatantly lied in his denial to the council.

Speaking to reporters, she also ordered Khartoum to halt all military operations against South Sudan and urged Juba to be restrained and not let itself to be provoked by the north.

In Geneva, U.N. human rights chief Navi Pillay called for an investigation into the bombing, saying the information available so far suggested that the attack may be an international crime or serious human rights violation.

There needs to be an independent, thorough and credible investigation to establish the precise circumstances of this aerial bombing, Pillay said.

If indeed it is established that an international crime or serious human rights violation has been committed, then those responsible should be brought to justice, she said.

UNMISS chief Hilde Johnson told the council via video link that on Thursday two bombs fell inside the Yida camp and three outside it. She said a local official in South Sudan had told UNMISS that more bombs may have been dropped on Friday, an allegation the U.N. mission was attempting to verify.

The number of casualties has yet to be confirmed, Johnson said, adding that a number of people were unaccounted for.

Ladsous said UNMISS has relocated at least 12 aid agency and U.N. staff from the area, which is near South Sudan's border with Sudan, and additional relocations are ongoing.

Violence around the poorly defined border has heightened tensions between Sudan and South Sudan since the south declared independence in July.

South Sudan split off into Africa's newest nation after voting for independence in a January referendum. The plebiscite was promised in a 2005 peace deal that ended decades of devastating civil war.

Some 2 million people died in the conflict, waged over ethnicity, ideology, religion and oil.