“The Rwandan defence forces have this time decided to violate … our territorial integrity by occupying the border town of Bunagana,” the military said in a statement, adding that it constituted “no less than invasion of the Democratic Republic of Congo”.

There was no immediate reaction from the government of Rwanda, but the government there has strongly denied accusations over the years that is supports the Congolese rebel group. Many of the M23 fighters are Congolese ethnic Tutsis and Rwanda’s president is of Rwandan Tutsi descent.

In a statement Monday, a spokesman for the rebel group called on Congolese President Felix Tshisekedi to open direct negotiations with them and said they seized the town only to make it safe enough for civilians to return after they fled recent violence.

Belgium’s king has regrets, but makes no apology for colonial past in Congo

“In the event of a new threat against our positions or the civilian population, our movement’s troops have received the order to follow and annihilate the threat no matter where it comes from,” M23 spokesman Willy Ngoma said in the statement.

Relations between Rwanda and Congo have been fraught for decades. Rwanda alleges that Congo gave refuge to the ethnic Hutus who carried out the 1994 Rwandan genocide that killed at least 800,000 ethnic Tutsis and moderate Hutus. The two countries have long accused each other of supporting various rival armed groups.

Late last month, Rwanda’s military accused neighbouring Congolese forces of injuring several civilians in cross-border shelling.

The M23 rose to prominence more than a decade ago when its fighters seized Goma, the largest city in Congo’s east which sits along the border with Rwanda. After a peace deal, many of M23’s fighters were integrated into the national military.

Then earlier this year the group appeared to make a comeback, launching an offensive against Congo’s military after saying the government had failed to live up to its decade-long promises.

The key town that was seized Monday, Bunagana, is only 60km (37 miles) northeast of Goma, which also serves as a hub for international aid organisations and the UN peacekeeping mission known as MONUSCO.

Bunagana, near the border with Uganda, is also an important transit point for goods being imported into Congo from as far away as China.

A spokesperson for UN Secretary-General Antonio Guterres said he was concerned about deteriorating security in eastern Congo, including M23 attacks.

Children among dozens killed in DR Congo machete massacre

African Union Commission Chair Moussa Faki Mahamat called for an immediate cessation of hostilities and for talks between Congo and Rwanda to resolve the growing diplomatic crisis.

The fighting caused more than 30,000 Congolese asylum seekers and 137 Congolese soldiers to cross into Uganda on Monday, Shaffiq Sekandi, Uganda’s resident district commissioner for Kisoro district, told Reuters.

“They are all over, the streets are full, others have gone to churches, they are under trees, everywhere. It’s a really desperate situation,” he said.

While the rebels claimed they took the town of Bunagana in order to stabilise it, local leaders on Monday urged Congo’s military to reclaim it.

“We deplore the M23 rebel attack and call on the Congolese government to track down and neutralise these rebel groups so that state authority can return,” said Innocent Ndagije, a civic leader in Bunagana.

Additional reporting by Reuters