(Reuters) - Rwandan-backed M23 rebels advanced on Sunday into the centre of Bukavu in eastern Democratic Republic of Congo, meeting little resistance from government troops, witnesses and the armed group said, as Congo accused Rwanda of ignoring calls for a ceasefire.

Rebel commander Bernard Byamungu told Reuters the M23 has had the city under its control since the middle of the day.

The rebels have been seeking to seize the commercial hub since taking the city of Goma in late January. Bukavu's apparent capture deals a further blow to Kinshasa's authority in the east of the country.

The Congolese government said rebels were in Bukavu and accused Rwandan troops of also entering the city. It stopped short of saying the city was fully under M23's control.

"Rwanda is stubbornly pursuing its plan to occupy, pillage and commit crimes and serious human rights violations on our soil," the government said in a statement.

The rebels' rapid advance since the start of the year and the involvement of troops from neighbouring countries have stoked fears of a broader regional war, rooted in tensions over power, identity and resources dating back to the 1990s Rwandan genocide.

Rwanda rejects allegations from Congo, the United Nations, and Western powers that thousands of its troops are fighting alongside the Tutsi-led M23. It says it is defending itself against the threat from a Hutu militia, which it says is fighting with the Congolese military.

"The entrance of M23 and the Rwandan Defence Force into Bukavu​ ... is a serious escalation that heightens the risk of a wider regional conflict – the human cost of which would be devastating​," Britain's foreign ministry said in a statement.

On Sunday, there was little sign of the clashes and bloodshed that preceded the capture of Goma, when about 3,000 people were killed, according to the United Nations.

Bukavu residents said they had seen M23 troops passing through central districts since the early morning with no sign of opposition.

M23 spokesperson Willy Ngoma posted a photo on X of a group of fighters posing in the provincial capital's central Independence Square.

The group's recent gains in North and South Kivu provinces mean it has captured more territory than all other rebellions since the end of two major wars that ran from 1996 to 2003. These advances have also handed them control of some of the region's valuable mineral deposits.

Congo is the world's top producer of cobalt, a key component in batteries for electric vehicles and mobile phones. It is also the third largest global copper producer and home to significant coltan, lithium, tin, tungsten, tantalum and gold deposits.

CONGO SOLDIERS WITHDRAW

Videos verified by Reuters showed M23 fighters in fatigues parading and speaking to locals in Bukavu's Independence Square and outside the provincial governor's office. Some locals welcomed the rebels with cheers and applause.

While the situation appeared largely calm, verified footage showed smoke rising from Bukavu's central prison.

Congolese troops have withdrawn from the city to avoid fighting in densely populated areas, South Kivu Governor Jean-Jacques Purusi told Reuters in a message, in which he also confirmed M23's arrival in Bukavu.

"They're in my residence already ... and my office," he said, adding that he and his team were safe elsewhere.

The gradual departure of Congolese and allied Burundian troops from Bukavu in recent days led to a power vacuum, triggering looting and a prison break on Saturday.

M23's arrival was welcomed by some in Bukavu, who hope the rebels will restore order, said Congolese researcher Josaphat Musamba, who is from South Kivu.

Yet the insurgency has deepened an already dire humanitarian crisis in eastern provinces. The "rapidly deteriorating" situation has left about 350,000 displaced people with no roof over their heads, the U.N. refugee agency warned on Friday.

The well-equipped M23 is the latest in a long line of ethnic Tutsi-led rebel movements to emerge in Congo's volatile east. It denies Congo's allegations it is a Rwandan proxy.

(Reporting by Congo Newsroom and Sonia Rolley in Paris; Additional reporting by Aaron McNicholas in London, Vinaya K in Bengaluru, and Cooper Inveen in Dakar; Writing by Alessandra Prentice; Editing by Helen Popper and David Holmes)