The Rwanda-backed M23 group has in recent weeks seized two major cities in eastern Democratic Republic of Congo (DRC), giving the armed group a major foothold in the mineral-rich region since taking up arms again in late 2021.

The group has continued its advance and the fighting has moved closer to the Burundian border, with the small Great Lakes nation -- which has supported Kinshasa with some 10,000 troops under a previous agreement -- increasingly sounding the alarm.

The UN authorised the evacuation of Bujumbura-based families of its staff last week, according to a letter dated February 21 from the UN's Department of Safety and Security and seen by AFP.

Several flights have been chartered since then, a UN employee with knowledge of the departures told AFP Thursday on condition of anonymity.

"Others are expected... to bring these families and certain non-essential personnel to safety," the employee added.

"We are acting with the greatest discretion so as not to upset the government," the source said, stressing these are "instructions that apply everywhere... when there are security risks".

On Thursday, Burundian President Evariste Ndayishimiye -- who previously warned against the threat posed by Rwanda -- adopted a more conciliatory tone.

"We are still committed to resolving the differences between our two countries by peaceful means," he told the diplomatic corps in Burundi's capital of Gitega.

However, his statement came as military and local sources said Burundi had placed troops on its border, some 10 kilometres (six miles) from Kamanyola town and the M23 advance.

Burundian military sources told AFP earlier this month troops were being withdrawn, although this was denied by officals.

Burundi is also seeing the largest influx of refugees in 25 years, with more than 43,000 people crossing the border in the last two weeks, the UN said.