At least two people died as crowds went on the rampage in the South Sudanese town of Wau on Wednesday, setting buildings on fire and forcing many to seek shelter with U.N. peacekeepers, witnesses said.

"I saw two people dead. One with a large wound in his chest, and one that had been cut into pieces using pangas (machetes)", said local journalist Deng Dimo.

Heavy gunfire was reported Wednesday as crowds rampaged through the South Sudanese town of Wau, setting buildings on fire and forcing many to seek shelter with U.N. peacekeepers, witnesses said.

"There is so much firing and so many houses are on fire, I see so many buildings in flames," said Bible Manding, a civil society activist in Wau, the state capital of Western Bahr-el Ghazal.

The head of Sudan's opposition political alliance, freed after two days in detention, said on Sunday that the government fears rising popular discontent in the crisis-hit nation.

Farouk Abu Issa, who represents more than 20 opposition parties, told Agence France Presse he was released on Saturday by the National Intelligence and Security Service (NISS).

The African Union on Saturday called for urgent talks between Sudan and South Sudan over the flash point Abyei region but backed off from a threat to refer the matter to the UN Security Council.

The AU's Peace and Security Council had given the two countries until December 5 to settle the final status of oil-producing Abyei, which Sudanese troops occupied for a year until May.

The International Criminal Court prosecutor may seek new war crimes charges against leading Sudanese officials over the Darfur conflict, she said Thursday.

Prosecutor Fatou Bensouda told the U.N. Security Council that aerial bombardments of civilians, deadly attacks on U.N. peacekeepers and humanitarian aid convoys "may constitute war crimes, crimes against humanity, and genocide."

Funding for the recovery of Sudan's war-ravaged Darfur region is in jeopardy unless the government eases restrictions on international aid personnel, a senior U.S. envoy said on Wednesday.

Dane Smith, the U.S. administration's senior adviser for Darfur, also said militias are "seemingly out of control" and have been implicated in attacks on peacekeepers, yet the government shows little interest in prosecuting the culprits.

Sudanese authorities have used excessive force against Darfuri students protesting for their rights, the U.S. government's senior adviser for Darfur said on Wednesday.

"We've also been very unhappy about the excessive force used against Darfuri students demonstrating for their rights under the agreement," Ambassador Dane Smith told reporters, referring to a 2011 peace agreement signed between Khartoum and an alliance of Darfur rebel splinter factions.

Sudan has carried out indiscriminate aerial bombing and other serious violations of international humanitarian law in its South Kordofan and Blue Nile border states, Human Rights Watch charged Wednesday.

The New York-based watchdog called for a U.N.-mandated investigation and sanctions on those responsible for the alleged violations.

Hundreds of Sudanese called for "revolution" on Monday, the second day of protests in support of four dead students originally from the conflict-plagued Darfur region.

Their deaths, following a crackdown on a tuition protest at Gezira University south of Khartoum, have sparked the largest outpouring of Arab Spring-style discontent since anti-regime protests in June and July.

The defense ministers of Sudan and South Sudan on Monday cut short talks on demilitarizing the tense border and other security issues but negotiations are going well, the South's minister said.

John Kong flew in from Juba on Sunday for talks with his Sudanese counterpart Abdelrahim Mohammed Hussein.