Uganda said on Monday it had returned dozens of former combatants of a Democratic Republic of Congo rebel group to a military camp after they tried to sneak back to their own country in disguise last week.

The former fighters, who have been staying in Ugandan camps after years of fighting in chaotic eastern Congo, were picked up in western Uganda. Congo has welcomed Uganda's recapture of the former combatants.

The M23 rebels, who once controlled swathes of territory in eastern Congo, fled to Uganda after a combined United Nations and Congolese force crushed their rebellion in 2013.

Arthur Timbaganya, a Ugandan regional military spokesman, said about 30 of the 100 or so people detained had turned out to be civilian Congolese refugees who had been staying in various camps. They were handed over to the U.N. refugee agency UNHCR, while "all the ex-combatants were returned to Bihanga (military camp)," he said.

The military has also increased its surveillance of the Bunagana area in southwestern Uganda, a border entry point with Congo, to try to prevent any more attempts by former M23 rebels to re-enter Congo, Timbaganya said.

"We put up roadblocks and we're checking vehicles," he said.

Any attempt by M23 to revive its rebellion would represent a new source of instability for Congo, where President Joseph Kabila faces mounting opposition after clinching a deal to stay in power beyond the expiry of his mandate last month.

On Jan. 15, Congo's information minister, Lambert Mende, said two columns of armed former M23 rebels had entered the country from Uganda before being "dealt with" by the Congolese military.

Timbaganya said he did not know why the former M23 combatants had tried to return to Congo, adding that the Congolese authorities probably had a clearer idea.

At its peak, M23 controlled Goma, capital of North Kivu province before its defeat and the flight of its fighters to Uganda and Rwanda, where they have been awaiting amnesties.

Many other armed groups remain active in eastern Congo.