Thousands of people have fled their homes to escape violence in the north of Central African Republic (CAR).
Aid agencies estimate that more than 7,000 refugees have crossed the border into Chad in the past few weeks.
A BBC reporter who visited the area says refugees claim government troops are systematically killing men and boys they suspect of backing rebel groups.
Central African Republic President Francois Bozize has blamed rebel groups for the unrest.
Hiding in the woods
The United Nations refugee agency is struggling to cope with the refugee influx into neighbouring Chad, and is warning the situation could become "catastrophic".
About 50,000 more refugees are thought to be hiding in the forest after being forced to flee their villages.
The BBC's Stephanie Hancock has been to the village of Bedakusan, in the border region of Chad, and says that for the last month the usually sleepy village has been home to more than 2,500 refugees fleeing the unrest.
The refugees were exhausted, many had walked for days through the bush to reach safety, our correspondent says.
They claim government troops are travelling from village to village in the north of CAR, entering villages and simply opening fire on anyone who is male.
They say that age is no barrier - many refugees told our correspondent they saw boys of just two or three years old shot dead.
Correspondents say international aid agencies have known for some time that a new human tragedy is unfolding in the north of the CAR.
But while the agencies can just about function in regions such as Darfur and eastern Congo, the level of insecurity in the northern CAR is so bad they cannot operate there at all, correspondents say.
Mr Bozize has blamed rebels opposed to his administration and bandits for the killings, but correspondents say the refugees are adamant that the culprits are government troops, decked out in their distinctive green berets.
Mr Bozize seized power three years ago, and since he stood successfully in a presidential election last year, a rebel movement has emerged in the north.