Rwandan rebels responsible for the 1994 genocide must surrender by year's end or face “military action,” the US warned on Tuesday.

“There is a tremendous need to finish off the FDLR,” said Russell Feingold, US special envoy for the Great Lakes region. He was using the French acronym for the Democratic Forces for the Liberation of Rwanda, which has been carrying out atrocities in the eastern Democratic Republic of Congo (DRC) since being forced out of Rwanda 20 years ago.

Ambassador Feingold said the FDLR must “meet the same fate” as the M23 rebel group and the Alliance of Democratic Forces — two other rebel groups that have operated in the eastern DRC.

M23, which the United Nations and DRC have accused Rwanda of supporting, was defeated earlier this year by “a combination of military and diplomatic efforts,” the envoy noted. A United Nations-sponsored combat brigade deployed in eastern DRC is credited with routing M23 on the battlefield. Bosco Ntaganda, the Congolese warlord from the rebel group surrendered himself for prosecution at the International Criminal Court (ICC) in March 2013.

READ: Uganda has stopped aid to M23, but Rwanda continues, UN says

The Alliance of Democratic Forces, originally comprised of Ugandans seeking to overthrow President Yoweri Museveni's government, “has been significantly diminished,” Feingold added. He offered a guardedly optimistic overall appraisal of developments in the DRC since taking up his portfolio in June 2013.

READ: Peace in DRC elusive as armed groups still thrive

“The situation in eastern Congo has been difficult and remains difficult but it is somewhat better than a year ago,” he said at a press briefing on the sidelines of the US-Africa summit. He said that FDLR fighters would be given a chance to demobilise, but that there had to be “significant surrenders in the near future or the military option must be exercised."

READ: Confusion clouds reports of FDLR ‘surrender’

Feingold also expressed concern about political developments inside Rwanda, Burundi and the DRC, saying that while Rwanda can claim very impressive achievements in development and economic growth, the US believes that “respect for human rights and opening space for opposition voices is essential.”

He confirmed that the US has had “candid conversations” with President Paul Kagame regarding Rwanda's record on press freedom and democratic rights, saying the issues were raised in talks with Kagame in Washington on Monday.

The US has also urged DRC President Joseph Kabila to respect the two-term limit enshrined in his country's constitution. US Secretary of State John Kerry reminded President Kabila of that obligation in a meeting in Washington on Monday, but the DRC leader offered no assurance that he would abide by the term limit provision. Mr Kabila's second five-year term is scheduled to end in 2016.

Feingold also offered an assessment of political conditions in Burundi that was similar to his comments on Rwanda. “We have seen progress over the years after some extremely tragic events,” he told reporters. But he also pointed to “growing constriction of political space” in the run-up to national elections scheduled for next year.