WASHINGTON - The United States warned Tuesday that the last major illegal armed group in eastern Democratic Republic of Congo must surrender in “significant” numbers soon or face military action to eliminate it.

Russ Feingold, the special US envoy to Africa’s Great Lakes Region, said officials from the region will meet at the United Nations Thursday to discuss how to proceed against the Democratic Forces for the Liberation of Rwanda (FDLR), a rebel group formed by Hutus who fled Rwanda in 1994. “There have to be significant surrenders in the near future or the military option must be exercised,” Feingold said on the sidelines of a US-Africa summit in Washington. He 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.”

It had promised to surrender May 30 but fewer that 200 fighters have since laid down their arms and the process has stalled.

“The United States strongly believes the military option with regard to the FDLR has to be not only on the table, but ready to be used,” Feingold said.

“If there is in fact a legitimate desire on the part of the FDLR to surrender, we are trying to see if that is true and to make sure those surrenders occur quickly.”

Two other armed groups - the M23 and the Allied Democratic Forces (ADF) - have been active in mineral-rich eastern DRC since the 1994 genocide of Tutsis by Hutus in neighboring Rwanda.

The M23, a Rwandan Tutsi rebel group, laid down its arms last year after a crushing defeat by the UN-backed Congolese army.

Feingold said M23’s reintegration was “well underway” and that the ADF had been “diminished” in strength.

Separately, US Secretary of State John Kerry met here with DRC President Joseph Kabila Monday to press for movement on a calendar for provincial and presidential elections in 2016, which Feingold said was crucial to stabilizing the region.

Under the DRC’s constitution, Kabila cannot run for re-election because he is limited to two consecutive terms, a provision which Feingold said was “unamendable.”

“We believe that it is in the interest of the DRC and its future reputational role that that be respected,” he said.

“I did not hear any assurances from either of them yesterday,” he said of the Kabila-Kerry meeting. “I believe the constitution should be respected and we are hopeful that is what will happen.”