The Human Rights Watch report, based on a January-February investigation in eastern Chad, documents a sharp increase in the number of attacks since December, resulting in dozens of civilian deaths and the displacement of tens of thousands of people.
"The government of Sudan is actively exporting the Darfur crisis to its neighbour by providing material support to Janjaweed militias and by failing to disarm or control them," said Peter Takirambudde, HRW's Africa director.
"The Janjaweed are doing in Chad what they have done in Darfur since 2003: killing civilians, burning villages and looting cattle in attacks that show signs of ethnic bias," he added.
The 15-page report, "Darfur Bleeds: Recent Cross-Border Violence in Chad," maintains that Sudanese forces have had a direct hand in the recent violence in Chad.
The report cites an example where Sudanese troops and helicopter gunships reportedly supported a Janjaweed attack across the border in the region of Goungor, Chad, on two occasions in early December.
HRW again called for the UN Security Council to urgently transfer the African Union force to a UN mission with a strong and clear mandate to protect civilians, by force if necessary.
HRW also called on the Security Council to place Janjaweed leaders responsible for the attacks on civilians in Chad on a list of people subject to travel bans and other UN sanctions.
Peacekeeping Criticism
Meanwhile the US Ambassador to the United Nations, John Bolton, has accused the UN and some Security Council members of moving too slowly toward setting up a UN peacekeeping force in Sudan's troubled Darfur region.
Ambassador Bolton expressed frustration with UN Secretary-General Kofi Annan and UN officials over the pace of preparation for the mission, which would replace 7,000 African Union troops.
He also said African and Arab diplomats on the Security Council needed to move more quickly.
"We're prepared, but the main thing, I think, is to get the internal UN operation to be moving more quickly, which we'd like to see," he said.
UN spokesman Stephane Dujarric said United Nations officials were in talks with African leaders about the force and that planning for the mission "is moving full steam ahead".