Angola on Friday released 55 soldiers from the neighbouring Republic of Congo who were captured earlier this week during an incursion, a Congolese official said.

The Congolese soldiers were captured on Monday in the Kimongo region by Angolan troops -- who had moved across the border the previous day -- and taken back into Angola.

"Our troops... have just been freed," Jacques Mouanda, a senior official from the Niari district where the incident happened, told AFP.

"We used diplomatic channels to secure their release. There were tough negotiations," he said.

The Angolan forces left Congolese territory on Thursday. A Congolese officer with the local paramilitary police said the Angolans claimed they were occupying a piece of land that belongs to Angola.

The Kimongo area has long been claimed by rebels in the neighbouring Angolan exclave of Cabinda and sources in the region said Angolan troops had crossed the international border in pursuit of the insurgents.

Cabinda borders on Congo to the north and is separated from the rest of Angola to the south by a sliver of territory belonging to the Democratic Republic of Congo.

Though small, Cabinda accounts for much of Angola's oil production, which makes the country the second largest producer in sub-Saharan Africa, behind Nigeria.

Cabinda rebels have been battling Angola since the southwestern African nation achieved independence from Portugal in 1975.

The Republic of Congo, Angola and the Democratic Republic of Congo have signed a non-aggression pact and, in 1997, Angolan troops helped Brazzaville's President Denis Sassou Nguesso to fight his way back to power in a civil war.