Rwandan-backed fighters closed in on eastern Congo’s largest city of Goma on Sunday, forcing thousands of civilians to flee and grounding flights from the local airport as government forces battled to stop the rebels seizing the city.

The M23 rebel movement has made rapid advances this month in Democratic Republic of Congo’s mineral-rich but conflict-riven eastern borderlands, raising fears that the fighting could spill over into a regional war.

The UN Security Council was due to meet a day earlier than planned on Sunday to discuss the crisis, diplomats said, after three UN peacekeepers – two South Africans and a Uruguayan – were killed.

Seven other South African soldiers and three from Malawi serving in a separate Southern African mission were also killed this week, South African and UN authorities said.

Three years into their current insurgency, the rebels, who the United Nations says receive backing from neighbouring Rwanda, now control more Congolese territory than ever before. Kigali denies supporting the group.

M23 fighters began moving on Goma, the capital of North Kivu province and home to about one million people, earlier this week and have vowed to seize the city. Gunfire and artillery fire could be heard on the city’s outskirts from early on Sunday, leading to panic in some areas, residents said.

By mid-afternoon, the rebels were approaching Goma’s airport, two government soldiers said.

“We’re at the Goma airport awaiting the attackers who are two or three kilometres (1 to 2 miles) from here,” one of the soldiers said.

Another Congolese military source, who asked not to be named, said government forces were “holding firm”.

Officials at the airport said flights were no longer operating. The UN on Sunday told staff in Goma not to go to the airport and to stay sheltered.

Eastern Congo remains a tinder-box of rebel zones and militia fiefdoms in the wake of two successive regional wars stemming from Rwanda’s 1994 genocide. Well-trained and professionally armed, M23 – the latest in a long line of Tutsi-led rebel movements – says it exists to protect Congo’s ethnic Tutsi population.

Congo’s government, however, says the rebels are proxies for Kigali’s expansionist ambitions in the region, an accusation the Rwandan government has long denied. Congo severed all diplomatic ties with Rwanda amid this week’s rebel offensive.

Congo’s army said on Saturday Rwandan snipers were responsible for the killing of North Kivu’s military governor on the frontline a day earlier. A spokesperson for Rwanda’s government did not respond to a request for comment.

Aid agencies are concerned about the conflict’s impact on civilians, warning that the fighting will deepen what is already one of the world’s worst humanitarian crises.

Several hundred thousand people had been forced to flee multiple zones of fighting since the latest M23 offensive began around Goma on Janary 23, the office of the UN humanitarian coordinator said in a statement.

“Several sites on the outskirts of Goma, sheltering more than 300,000 displaced people, were completely emptied in the space of a few hours,” the statement said.

The escalation in violence has forced the World Food Programme to temporarily pause emergency operations, its executive director said in a social media post on Sunday.

• A Reuters report