Janjaweed militiamen killed up to 400 people in the volatile eastern border region near Sudan, leaving an "apocalyptic" scene of mass graves and destruction, the U.N. refugee agency said Tuesday.

The attacks took place March 31 in the border villages of Tiero and Marena, some 550 miles from the capital, N'djamena. Chadian officials initially said 65 people had died, but said the toll was sure to rise.

"Estimates of the number of dead have increased substantially and now range between 200 and 400," the U.N. High Commissioner for Refugees said in a statement. "Because most of the dead were buried where their bodies were found — often in common graves owing to their numbers — we may never know their exact number."

The attackers encircled the villages and opened fire, pursuing fleeing villagers, robbing women and shooting the men, UNHCR said. Many who survived the initial attack died later from exhaustion and dehydration, often while fleeing.

Sudan and Chad repeatedly have accused one another of backing rebels in each other's countries, and both have denied the allegations. Both countries also have signed peace deals promising to stop the border fighting.

U.N. officials have warned of the possibility of increasing violence in the region where Chad, Sudan and the Central African Republic meet. Fighting in Sudan's Darfur region reportedly has left more than 200,000 people dead.

Sudanese leaders are accused of unleashing the pro-government Arab militia, the janjaweed, which is blamed for widespread attacks and rapes against ethnic Africans.

On Monday, the Sudanese military said 17 of its soldiers were killed repelling a Chadian army raid on a Sudanese border town in West Darfur.

Sudanese army spokesman Osman Mohamed al-Aghbash said in a statement issued by the military that a company of the Chadian army, consisting of 140 trucks and seven armored vehicles, crossed into Sudan and attacked For Baranga, a small town within several kilometers (miles) of the Chad border.

He said the Sudanese army and police pushed the attackers back across the border, but 17 soldiers were killed and 40 wounded in the process.