A mysterious armed group has seized a mining town in the south-east of the Democratic Republic of Congo.
The attack on Kilwa in the mineral-rich Katanga province by the group known as the Liberation Movement for Katanga has prompted a reported evacuation.
Congolese troops are being re-grouped to re-take the town, officials said.
Meanwhile, President Joseph Kabila is due to go to Kisangani on his first visit to the volatile east of the country since coming to power in 2001.
During the five-year civil war, which ended last year, eastern towns including Kisangani were controlled by rebels and cut off from the capital.
Observers say the visit is being seen as a significant symbol of the continuing re-unification process.
The BBC's Robert Walker in Kisangani says there is a huge sense of expectation and excitement, with thousands of people expected to line the streets to greet him.
However, our correspondent says the visit also highlights the challenges faced by President Kabila's government.
Tens of thousands of people are still displaced, and other parts of the east remain unstable, our correspondent notes.
Little is known about the occupiers of Kilwa, which lies in a region largely spared of the fighting elsewhere in the country.
Government and diplomatic sources report that some of the troops entered from neighbouring Zambia and speak Portuguese - which has led to speculation that they might have come originally from Angola.
Katanga was largely spared the fighting seen elsewhere
A Congolese envoy has been sent to Zambia to discuss the matter and government troops were being re-grouped, Information Minister Mova Sakani said.
Kilwa is located near an important cobalt and copper mine, which is exploited by an Australian company.
Some foreign aid workers and mining experts have been evacuated, according to diplomatic sources.
The BBC's Arnaud Zajtman in the capital, Kinshasa, says Katanga has continued to attract foreign businessmen, but this military development is a setback for Congolese economic development.