BUKAVU, Democratic Republic of Congo (Reuters) - Naval boats fended off attacks by rebels on Lake Tanganyika in the east of Democratic Republic of Congo on Thursday as heavy fighting spread close to the lakeside city of Uvira, sources in the area said.

Fighting between the Mai-Mai Yakutumba militia and Congo government forces broke out at the weekend on the outskirts of Uvira, close to the Burundi border. Unrest has mounted across Congo since President Joseph Kabila refused to step down when his mandate expired last December.

“Since 5 a.m. (0300 GMT) there has been an exchange of gunfire between the army and the Mai-Mai in Uvira,” said Lubungula Dem’s M’Sato, a member of a peacebuilding advocacy group in Uvira, the second largest city in South Kivu province.

Congo’s navy also repelled an attack by five rebel boats on the lake, military spokesman Louis-Claude Tshimwanga said, adding that the navy had sunk one of them and that government forces remained in control of Uvira.

Lake Tanganyika is hundreds of kilometers (miles) long and also borders Burundi, Tanzania and Zambia. Mai-Mai Yakutumba, formed in 2007 by local militiamen opposed to integration in Congo’s national army, has well-established gold smuggling networks on the lake.

In an audio statement this week that was shared on social media, its leader William Yakutumba said his forces were rebelling against Kabila’s mismanagement of the country’s natural resources and failure to quit power last year.

Militiamen believed to be aligned with Yakutumba in a coalition opposed to the Kinshasa government also clashed on Thursday with army troops in Kasongo in neighboring Maniema province, said Modeste Shabani, the director of a local radio station.

Meanwhile, the Congolese army’s chief of staff, Didier Etumba, arrived in South Kivu’s capital of Bukavu in the afternoon en route for Uvira, a Reuters witness said.

Congo’s mineral-rich eastern borderlands are a tinderbox of ethnic tensions and for more than two decades have been racked by violence that has often spilled across the country’s borders. The region is the world’s biggest source of coltan, used in mobile phones and other electronic products.

Operations at Banro Corp’s Namoya gold mine near the border between Maniema and South Kivu remained suspended for a fourth day on Thursday due to nearby militia activity, Banro vice president Desire Sangare told Reuters. The company’s mines in the two provinces have been hit by a series of attacks and kidnappings in recent months.

Congo’s U.N. peacekeeping mission, MONUSCO, said it had deployed troops around Uvira to protect civilians although the recent elimination of a 100-person riverine unit as part of sweeping budget cuts across U.N. missions limited its options.

“I urge the armed groups to immediately cease this hostility including all forms of violence against constituted authority and innocent civilians,” MONUSCO head Maman Sidikou said in a statement.