The UN's special envoy in Sudan says government military forces have been operating in the western Darfur region.
Mr Pronk says more African Union troops are needed in Darfur
Jan Pronk told the BBC that an attack on the village of Hamada, which left 100 people dead, was a recent serious incident.
He said government bombers and helicopter gunships fly regularly over north and south Darfur and 40 villages had been hit by pro-government militia.
Sudan's government and Darfur rebels signed a ceasefire last year, but this
has been violated by both sides.
Mr Pronk told the BBC's Focus on Africa programme the African Union's 1,200-force in Darfur is doing an "excellent job" but needs more troops.
African Union monitors have been trying to investigate the reported air attack on the town of Shangil Tobaya on Wednesday, where 100 people are believed to have died.
They were turned away by Sudanese soldiers on Thursday, an AU official told the BBC earlier.
Mr Pronk said observers could not get to the area because it is not safe. "We know that there are at the moment government military trucks driving in that direction," he told Focus on Africa.
He said government-linked Janjaweed militia had attacked around 40 villages in the area around Labado in south Darfur.
"Many villages around Labado are constantly being attacked - not with airplanes but by militia and these villages are being burned down and completely demolished," he said.
"Where the AU are present, they do not attack. The AU is doing an excellent job at the moment but they don't have enough people."
He described the militia as "loyal to themselves" whose objective is to "drive out all people who do not belong to their own tribes - mostly they are African tribes who are being driven out."
He told the programme he believed plans to boost the AU force to 3,500 needed to be more than doubled to 8,000.
AU monitors are also investigating the attack on the nearby village of Hamada last week.
Some 9,000 people have fled the area, a UN official said.
'Genocide' findings due
A United States official said all sides were involved in the latest fighting.
"All the parties, the government of Sudan, the militias that are allied with the government and the rebels, are to blame for this increase in violence," said state department spokesman Richard Boucher.
The latest fighting has produced more refugees, the UN says
Some 70,000 people have been killed in the two-year conflict.
A further two million people have fled their homes, mostly black Africans, who accuse pro-government Arab militias of committing atrocities.
The Sudanese government denies backing the Janjaweed militias, and blames rebels for starting the conflict.
A UN investigation on whether genocide has been committed in Darfur has been completed and is expected to be made public next week.