Militants have killed two United Nations workers in Democratic Republic of Congo's restive north-west. Heavily armed rebels attacked the town of Mbandaka and overran the airport, UN officials said, killing a Ghanaian peacekeeper and another UN employee. Several civilians were also killed in the attack and UN peacekeepers and Congolese troops were still trying to retake the airport, officials said. It is not yet clear which rebel group was behind the attack. Mbandaka is the capital of Equateur province, where at least 100 people were killed in clashes between rival tribes late last year. The violence between the Lobala and Boba tribes started after a dispute over fishing rights and lasted for a month from October, displacing an estimated 200,000 people. At least 30 fighters were believed to have been part of the latest attack. The BBC's Thomas Fessy in Kinshasa says the joint operation to retake the airport, launched by Congolese and UN troops, has been suspended overnight, and should resume in the early hours of the morning, according to Gen Janvier Mayanga, the man in charge of government troops in the provincial capital. Madnodje Mounoubai, spokesman for the UN mission in DR Congo, told Reuters news agency it was thought the rebels were from the Enyele tribe - a Lobola sub-tribe. The UN is in talks with DR Congo officials on withdrawing its 20,500-strong peacekeeping mission, Monuc, the biggest UN operation in the world. The mission's current mandate expires in May. Our correspondent says this new fighting might put such a withdrawal into question. Source: BBC