(CNN) -- The U.N. refugee agency says "unknown armed elements" are thwarting its efforts to move Darfur refugees in eastern Chad from the "volatile border to camps."
"The situation is so serious that our representative in Chad is now at the border trying to find a solution to this problem, which is leaving the refugees extremely exposed and vulnerable," the U.N. High Commissioner for Refugees said in a news release.
"The area is highly insecure with roaming armed groups posing a real threat to the refugees and aid workers."
Chad is experiencing an influx of refugees as a new wave of displaced people flees from Sudan's Darfur region, which has endured years of violence.
Roughly 200,000 people have been killed and at least 2 million others have been displaced, with nomadic Arab Janjaweed militias -- allegedly allied with the Sudanese government -- targeting pastoral blacks.
An estimated 12,000 refugees from Darfur have streamed into Chad recently adding to the 240,000 in camps there already.
"We are very disturbed that in eastern Chad on Tuesday our efforts to move traumatized, newly arrived refugees from West Darfur away from the volatile border to camps was blocked by the presence of unknown armed elements," UNHCR said.
"They gave no reason for their actions but it was clear the relocation would not take place. This is deeply concerning and we are making every effort with the Chadian authorities to get these refugees moved quickly."
The agency says the refugees are "exhausted," women are reporting rape cases, and some children are missing.