Some 600 Congolese soldiers have fled into Uganda, following clashes with rebels who have seized a border town.
The Ugandan military said the fleeing troops had been disarmed.
As the rebels took control of the Democratic Republic of Congo side of the town of Bunagana, an Indian peacekeeper was killed, the UN says.
M23 rebels loyal to Bosco Ntaganda, who is wanted for war crimes by the International Criminal Court, took up arms in April.
They defected from the army after pressure increased on the government to arrest Gen Ntaganda, when one of his former colleagues was convicted of recruiting child soldiers by the ICC.