As an East African bloc urged an immediate ceasefire in eastern Democratic Republic of Congo (DRC), Rwandan-backed M23 rebels who seized the city of Goma extended their advance on Wednesday, and Congo said it planned a campaign to recover lost territory.

While saying he favours diplomacy, Congo President Felix Tshisekedi told the nation on Wednesday night that he would mount a military riposte and warned: "The presence of thousands of Rwandan soldiers on our soil ... (is leading) to an escalation with unpredictable consequences."

The eight countries of the East African Community held an emergency summit and called for an immediate ceasefire in eastern Congo and for Congo to negotiate with M23. The United Nations Security Council similarly demanded a halt to the rebel offensive earlier this week.

Congo and Rwanda are both members of the East Africa bloc along with Kenya, Tanzania and other states. Tshisekedi did not attend the summit while Rwanda's President Paul Kagame did.

M23 rebels captured Goma, a city of 2-million people and capital of North Kivu province, on Monday. Rwandan forces backed up M23 in Goma, according to Congo, the UN, the US and other Western powers. Rwanda has denied this.

The rebels advanced south on Wednesday in an apparent attempt to expand their area of control. They marched along the western side of Lake Kivu and neared Kavumu, where Bukavu's airport is located.

This week's events represent the gravest escalation of the decades-old conflict in eastern Congo since 2012. The hostilities are rooted in the spillover of Rwanda's 1994 genocide into Congo and struggle for control of Congo's lucrative minerals.

In Goma, a hub for displaced people, aid workers, UN peacekeepers and Congolese forces, the rebels were consolidating their hold on the city on Wednesday and patrolling the border with Rwanda.

A flurry of diplomatic activity, including the United States telling Rwanda it was "deeply troubled" by Goma's fall and Germany cancelling aid talks with Rwanda, was having no apparent effect on the ground.

MERCENARIES EXIT VIA RWANDA

Isolated gunfire sounded on Wednesday in some outlying districts of Goma. Bodies from Monday's battles lay in the streets, hospitals were overwhelmed and UN peacekeepers were sheltering in bases.

At a border crossing between Goma and its Rwandan twin city of Gisenyi, Reuters reporters saw dozens of Romanian mercenaries, who had been hired by Congo to bolster its defences, crossing the border into Rwanda -- the start of their journey home, one said.

Reported by Rwanda to number more than 280, they lined for police dogs to sniff their luggage and police to check documents and pat them down. They then boarded coaches to Kigali.

M23 is the latest ethnic Tutsi-led, Rwandan-backed insurgency to fight in Congo since the aftermath of the genocide 30 years ago, when extremist Hutus killed Tutsis and moderate Hutus, and then were toppled by Tutsi-led forces led by Kagame. Kagame has been Rwanda's president ever since.

Rwanda says some of the ousted perpetrators have sheltered in Congo since the genocide, posing a threat to Congolese Tutsis and Rwanda itself. Congo rejects Rwanda's complaints, and says Rwanda has used its proxy militias to loot its minerals.

In an interview with Reuters on Wednesday, Rwanda's foreign minister called for a ceasefire throughout eastern Congo and urged Kinshasa to negotiate with the rebels.

The East African leaders said they plan to hold a joint summit on the crisis with southern African leaders in the coming days. Tshisekedi on Wednesday visited neighbouring Angola, which has mediated in the crisis, Angola's presidency said.