Tribal violence and rebel-government battles have displaced at least 460,000 people in Sudan's Darfur this year, the United Nations said on Thursday, as dozens reportedly died in the latest clashes.

"According to humanitarian organizations, so far in 2013 at least 460,000 people have fled their homes in Darfur as a result of inter-tribal fighting and clashes between the SAF (Sudanese army) and armed movements," the U.N. Office for the Coordination of Humanitarian Affairs said in its weekly bulletin.

Sudanese rebels on Tuesday claimed their first major attack of the new fighting season, even as the country's defense minister vowed to crush them in coming months.

There were "many, many casualties, many dead bodies" in the afternoon ambush which destroyed three tanks near Dilling town in South Kordofan state, said Arnu Ngutulu Lodi, spokesman for the rebel Sudan People's Liberation Army-North (SPLA-N).

Thousands of Sudanese demanded the removal of a local police chief on Tuesday, witnesses said, on the third day of unrest sparked by an attack on a female student.

Crowds in Abu Hamad, about 500 kilometers (300 miles) north of Khartoum, "started a sit-in near the main court," one witness told AFP.

Tribal violence has eclipsed rebel activity as the major security threat to Sudan's Darfur, the defense minister said Tuesday, vowing nonetheless to crush the rebels there and in South Kordofan.

Battles between Arab tribal militias have worsened this year in Darfur, leaving hundreds dead in the country's far-west region, where a decade-long rebellion also continues.

Tribal violence and not rebel activity is the major security threat to Sudan's Darfur region, the defense minister said on Tuesday.

Battles between Arab tribal militias have worsened this year in Darfur, leaving hundreds dead in the country's far-west region, where a decade-long rebellion also continues.

The Sudan government and a key rebel group are refusing to let U.N. workers vaccinate 160,000 children against polio in conflict-stricken states despite agreeing to a ceasefire, the U.N. said Monday.

U.N. humanitarian operations director John Ging said he had appealed to the U.N. Security Council for pressure to end what he called a "filibuster" by the Sudan government and opposition Sudan People's Liberation Movement-North (SPLM-N) on humanitarian access.

Arab tribal militias fired rockets, artillery and heavy machine guns during battle across a wide swathe of Sudan's South Darfur on Sunday, sources on both sides said.

They said they could not yet give a casualty count, but tribal unrest which worsened this year in Darfur has already killed hundreds.

Dozens of government troops have been killed during a rebel ambush in Sudan's Darfur, a regional analyst said on Thursday, confirming an unusually high toll for the troubled region.

There were "huge" casualties on the government side, numbering in the "dozens", although exact figures were unclear, the analyst told Agence France Presse on condition of anonymity.

Four Ethiopians were killed in a bomb blast on a bus near the Sudanese border, the information minister said Thursday, as security forces remain on high alert following attack warnings.

"One minibus was blown up on the road and four people were killed," Information Minister Redwan Hussein told AFP, adding it was not known who the attackers were.

Sudanese officials in a region bordering Eritrea told European diplomats Wednesday that they need help to combat human trafficking, an issue highlighted when hundreds of Eritrean migrants died off Italy.

"We are confronted by organised (criminal) groups," the governor of Kassala state, Mohammed Yousef Adam, told European Union ambassadors visiting from Khartoum.