By Michelle Nichols

UNITED NATIONS (Reuters) - U.N. peacekeepers found an arsenal of weapons, including heavy caliber arms and enough ammunition to last a year, at the site of the last stand by M23 rebels in eastern Democratic Republic of Congo, U.N. peacekeeping chief Herve Ladsous said on Friday.

Congolese troops and the U.N. peacekeeping mission - which includes a unique Intervention Brigade mandated to eliminate armed groups - last month defeated M23, which signed a peace deal with the Congo government on Thursday.

The Tutsi-led M23 rebel group ended its 20-month rebellion, the most serious in Congo in the last decade, after Congolese soldiers and U.N. peacekeepers, known as MONUSCO, captured its last hilltop strongholds in Chanzu, near the Rwandan border.

"I flew over the hill of Chanzu, which was the last battle effectively between FARDC (Congolese troops), helped by MONUSCO, and M23," Ladsous told reporters.

"It's amazing the quantity that were seized there, including heavy caliber, hundreds of tonnes really of all sorts of stuff including about a year's worth of ammunition supplies," he said.

Ladsous said a U.N. Group of Experts, who monitor compliance with sanctions and an arms embargo on Democratic Republic of Congo, was investigating the origin of the weapons, adding: "I'm sure it will make interesting reading."

The U.N. experts, who report to the U.N. Security Council's Congo sanctions committee, have repeatedly accused neighboring Rwanda of backing the rebellion by M23 in eastern Congo, a claim the Rwandan government has fiercely rejected.

Western officials say that Rwanda's denials are not credible, and U.S. and European governments imposed punitive measures on Kigali to pressure it to halt its support for M23.

Rwanda has repeatedly intervened in Congo, saying it had to hunt down the Hutu militia who fled after the 1994 Rwandan genocide. Rwanda and Congo have fought two wars over the past two decades in Congo's resource-rich east.

THOUSANDS SURRENDERING

Rwanda has accused Congolese troops of collaborating with the Democratic Forces for the Liberation of Rwanda (FDLR), a charge Kinshasa has denied. Hutus - who fled Rwanda after the genocide of 800,000 Tutsi and moderate Hutus - make up about 30 percent of the FDLR fighters, according to the United Nations.

Following the defeat of M23 last month, Congolese troops and U.N. peacekeepers have now turned their attention to combating the FDLR Hutu militia.

Ladsous said that some 2,300 rebel fighters in eastern Congo from several groups, including M23, have given up their weapons to U.N. peacekeepers in the past two months or so.

"We are seeing those relatively large numbers of people just stop fighting," he said. "I think it is in large part because of the deterrent effect of the Intervention Brigade and now the drones."

During his visit to eastern Congo earlier this month, Ladsous helped launch the United Nations first unmanned surveillance drones. He said two were in use now and that number would increase to five by April.

Thick forests, rugged terrain and the scarcity of roads on Congo's eastern border with Rwanda and Uganda have complicated efforts by the U.N. peacekeeping force to control the resource-rich area.

Millions of people have died from violence, disease and hunger in eastern Congo since the 1990s while dozens of rebel groups have fought for control of its rich deposits of gold, diamonds, copper, cobalt and uranium.

If the trial surveillance in eastern Congo is successful, officials and diplomats also hope the drones could be used by other peacekeeping missions in Ivory Coast and South Sudan.

(Reporting by Michelle Nichols; Editing by Diane Craft)