The U.N.’s top human rights official called Friday for an “independent, thorough and credible investigation” into allegations that Sudan’s air force had bombed a refugee camp in South Sudan.

“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,” said Navi Pillay, the high commissioner for human rights.

The United States pressed the U.N. Security Council to adopt a statement condemning Sudan’s government for the attack, which U.S. officials said was the first aerial bombing campaign in South Sudan since the two countries split up in the summer.

“We condemn in the strongest possible terms the aerial bombardment of targets by the north in South Sudan — including the most outrageous, which was the bombing of Yida refugee camp, housing 20,000 people, repeatedly by air over the course of several hours,” Susan E. Rice, the U.S. ambassador to the United Nations, told reporters.

After a tense public meeting on Sudan, Rice said her Sudanese counterpart had “blatantly lied” to the council by denying that his government attacked any targets in South Sudan. The United States maintains that Sudan has bombed at least two other targets in South Sudan in recent days.

Sudan’s U.N. envoy, Daffa-Alla Elhag Ali Osman, denied that his government had carried out any aerial attacks in South Sudan, dismissing witness accounts reported by foreign news media as “fabrications.” He also said there was “no refugee camp whatsoever in our border between the north and south.”

Osman accused South Sudanese authorities of supporting rebels in restive provinces of Sudan, including Southern Kordofan and Blue Nile.

South Sudan’s acting U.N. representative, David Buom Choat, rejected those allegations. “We have not supported anybody that is fighting Sudan,” Choat said. “But it has been the attitude of the Republic of Sudan to always blame others for their own problems.”

The top U.N. peacekeeping official, Herve Ladsous, said there were confirmed reports that one bomb was dropped near the camp, which was hosting at least 10,000 people who had fled fighting in Southern Kordofan, and another near a military encampment. There were unconfirmed reports that two additional bombs were dropped in the area, he said.

South Sudan formally became an independent nation in July, a culmination of decades of armed struggle against the north in which an estimated 2 million southerners died. Many issues between the two sides remain unresolved, and Ladsous expressed concern that the tensions have been building in recent months, particularly in the disputed region of Abyei.

“While neither side appears ready to return to war, there is a concerning pattern of escalation and destabilization that could lead to large-scale confrontation and put more lives at risk,” Ladsous said.