A leading human rights group says militias in Sudan's troubled Darfur region are launching raids on villages in Chad, threatening thousands of civilians, amid accusations of cross-border attacks from both sides. Human Rights Watch is pushing for swift action by the United Nations to authorize an expanded international force in Darfur and along the Chadian border.

The New York-based human rights organization says it has documented numerous cases of attacks since early December in eastern Chad in which civilians have been killed, villages burned and cattle stolen. The group says the raids had apparent Sudanese government backing, and were perpetrated by Sudanese and Chadian militiamen, based in Sudan's Darfur region.

The African Union's special envoy to Sudan, Baba Ghana Kingibe, said last week there is no evidence to support accusations by Chadian President Idris Deby that Sudan is arming and harboring Chadian rebels.

Both Sudan and Chad accuse each other of supporting rebel movements against the other.

The United States, which has taken over the rotating presidency of the United Nations Security Council for the month of February, says the Darfur situation is a priority. The top U.S. diplomat for Africa, Jendayi Frazer, acknowledged the recent escalation along the border.

"I think what we are seeing, particularly since the end of December and January, is there is obviously a lot more conflict taking place on the border between Chad and Sudan," said Frazer. "And the relationship between these two countries have complicated the security situation in Darfur, as the Chadian government has made charges that the Sudanese are supporting rebels in their territory, and counter charges [have been made] from the Sudanese."

Human Rights Watch says the situation is worrisome, and is calling for more international action.

"The situation is escalating, it has gotten much more serious in the past few weeks, and this requires much stronger international action to protect these thousands of civilians who are now threatened," added Georgette Gagnon, group's deputy director for Africa.

On Friday, the United States put in motion the first steps toward establishing a U.N. peacekeeping mission in the Darfur region. Washington won unanimous Security Council support for a statement calling on the U.N. secretariat to prepare for transferring the African Union force in Darfur, to U.N. control.

Gagnon of Human Rights Watch says that transition will take several months, and the African Union and the international community must take measures in the interim to protect civilians from violence.

"There are steps that the African Union force and international community can take to protect the people in Darfur, and also in Chad, from the escalating violence and attacks by these militias," said Gagnon. "Of course, the Sudanese government, as well, should be disarming and disbanding the militias that it sponsors, who are responsible for most of these attacks."

There was no immediate Sudanese government response to the Human Rights Watch report.

The human rights group says it has found that security for people in Eastern Chad and Western Darfur are closely connected, and if no preventive action is taken, it may only be a matter of time before the refugee camps in Chad are also threatened.

Three years of war in Darfur between rebel groups and government-backed Arab militias, have left tens of thousands of people dead and displaced some two million others.