Fighting between rebels and government troops in Chad has sent thousands of civilians, mostly women and children, across the border into Sudan's Darfur region, adding pressure to the refugee crisis in the area.

The UN High Commissioner for Refugees said 8,000 to 10,000 people have set up camp on the Darfur side of the frontier.

"Confirmation that Chadians have crossed into Darfur is certainly a worrisome new development and a sign of the deteriorating security situation across the border," high commission spokeswoman Jennifer Pagonis told reporters.

Human Rights Watch said Chadian and Sudanese militias based in Darfur were entering Chad, conducting deadly raids and displacing tens of thousands of people.

The refugees fleeing the fighting in Chad provide "further evidence of the spreading insecurity that now straddles this increasingly insecure region," Jennifer Pagonis, a spokeswoman for the UN high commissioner for refugees, told reporters.

About 300,000 refugees have fled to Chad to escape the conflict between rebels, and Sudanese government forces and militias in Darfur.

Sudan has accused Chad of harbouring Darfur rebels, while Chad has said Sudan backs Chadian insurgents.

"You may have thought the terrible situation in Darfur couldn't get worse, but it has," Peter Takirambudde, African director of Human Rights Watch, said in an early February statement.

"Sudan's policy of arming militias and letting them loose is spilling over the border, and civilians have no protection from their attacks, in Darfur or in Chad."

Tens of thousands of Darfur residents have died and more than 1.2 million have been displaced from their homes in the wake of the violence.

Critics say the Sudanese government has been supporting Arab militias accused of attacking Darfur's black residents and carrying out a brutal campaign to drive out the local population. The government denies the charge.