The UN mission chief in the Democratic Republic of Congo on Thursday warned the last remaining rebel group in the east to lay down arms or face military action.

The FDLR, a Rwandan Hutu militia that includes perpetrators of the 1994 genocide of Tutsis, has been given a six-month deadline to disarm and to hand over those wanted for war crimes, Martin Kobler told the UN Security Council.

"To the FDLR: Seize this last opportunity and move forward with this process. All combatants, including leaders, must disarm now," said Kobler.

"I suggest joint military actions against those FDLR factions not willing to disarm."

Disarming the FDLR, of which only 1,500 combatants remain, took on added urgency following the defeat of the mainly Tutsi M23 rebels in the eastern DR Congo late last year.

FDLR military commander Sylvestre Mudacumura is wanted by the International Criminal Court for war crimes, including torture, in connection with a murderous rampage in North and South Kivu in 2009 and 2010.

In May, the Kinshasa government presented a plan to resettle the remaining FDLR fighters outside the country, giving them 22 days to turn up at two camps from where they were to be taken to a third site for their relocation.

A total of 186 combatants and 430 dependents took the government's offer but the plan has since stalled.

Several UN, Kinshasa and other regional envoys have sought to convince the FDLR leaders to move forward, but they are refusing to give the order to move to the third site, according to Kobler.

- Slow progress -

Angolan Defence Minister Joao Manuel Goncalves Lourenco, whose country wields influence in the region, said he was "concerned by the slow progress of the voluntary surrender of the Democratic Forces for the Liberation of Rwanda."

Angola is due to host a summit next week that is expected to see leaders decide on a tougher stance to disarm the FDLR.

The US special envoy to Africa's Great Lakes region, Russ Feingold, called for flushing out the FDLR during a US-Africa summit in Washington this week.

Feingold said the United States believes there is "no justification" for the FDLR's demand for a political negotiation, and that the group should demobilize "no later than the end of the year."

Two FDLR leaders, Ignace Murwanashyaka and his deputy Straton Musoni, went on trial in Germany in 2011, charged with mass killings and rape in the eastern DR Congo. A verdict has yet to be handed down.

- UN exit strategy -

DR Congo Foreign Minister Raymond Tshibanda told the Council it was time for the 20,000-strong UN mission MONUSCO to wind down, allowing the UN to focus on troublespots elsewhere.

"Considering the fewer security risks and progress accomplished by the strengthening of our national capacity in all sectors, I call on behalf of the government of the Democratic Republic of Congo, for a substantial reconfiguration and downsizing of the MONUSCO force," said Tshibanda.

The UN mission chief said, however, that there were no immediate plans to scale back the operation, although UN peacekeeping officials are looking at an "exit strategy."

Questions about MONUSCO, the UN's biggest mission and operating in the DR Congo for 20 years, are being raised as the UN prepares to launch a new peacekeeping mission in the Central African Republic in September.