Rebel fighters have partially captured the southern city of Bangassou in the Central African Republic (CAR), weeks after they were accused of an attempted coup and ahead of partial results from a tense presidential election.

A coalition of armed rebel groups, which controls two-thirds of the coup-prone country, launched an offensive on December 19, aiming to disrupt last weekend’s elections and “march on [capital] Bangui”.

They have been kept away from the capital city so far by federal soldiers, United Nations peacekeepers and reinforcements sent from Russia and Rwanda. But the rebels attacked Bangassou, a diamond-mining town which sits on the border with the Democratic Republic of the Congo (DRC) about 750km (470 miles) from Bangui, at dawn on Saturday.

“The rebels control the town,” Rosevel Pierre Louis, head of the UN peacekeeping force MINUSCA’s regional office in the city, told AFP news agency on Sunday. “They are everywhere.” Government troops “abandoned their position and are at our base,” he added.

At least five rebels were killed and two army soldiers were wounded in the clashes, MINUSCA said in a statement, adding that the fighters were allied to former President Francois Bozize. Doctors Without Borders (Medecins Sans Frontieres, or MSF), said it had transported 15 wounded in the fighting.

The government of President Faustin Archange Touadera – the favourite to win the December 27 election – has accused Bozize of fomenting an attempted coup, a charge he has denied.

The rebels launched the offensive last month after the constitutional court rejected Bozize’s candidacy to challenge Touadera in the election. The rebels also waged a dawn assault on Saturday on the town of Damara, which is Touadera’s stronghold about 70km (45 miles) north of Bangui, but were repelled by troops, MINUSCA said.

The capture of Bangassou comes on the eve of partial results being published for the presidential election. Lack of security meant that voting did not take place in 29 of the country’s 71 sub-prefectures, while thousands were prevented from casting ballots or never received their voting cards.

Threats and attacks by the rebels kept more than 14 percent of polling stations closed on Election Day. Touadera’s party claimed victory last week, while a coalition of opposition groups called for the election to be annulled, claiming ballot stuffing and electoral rigging.