The United States exerted key pressure on Rwanda to halt its support for M23 rebels in the Democratic Republic of Congo, seeking to end the latest conflict in the Great Lakes region of Africa.

An offensive this week by Congolese soldiers backed by UN troops defeated the M23 rebels, who launched an uprising in the mineral-rich east of the country in early 2012.

Washington had already put its ally Rwanda on notice in July 2012, freezing annual military assistance of $200,000.

Then, a month ago, Washington announced sanctions concerning the training of Rwandan officers.

The goal was to get the government of President Paul Kagame to break completely with the Tutsi mutineers of the M23.

"It is clear that the US has been behind the scene putting more pressure on Rwanda," said Richard Downie of Washington's Center for Strategic and International Studies (CSIS).

He cited as evidence "the assistance over the border from Rwanda which was pretty significant until farly recently has trickled down to virtually nothing in the past few weeks."

Kagame took power after the 1994 genocide committed by Hutu extremists, and the United States has supported his Tutsi-led government for nearly 20 years.

But now, Downie said, Washington has "very slowly, but increasingly come around to the view that Rwanda is the critical piece of trying to resolve this chronic conflict and that Rwanda's role has been frankly very unhelpful."

After long keeping quiet, in recent months Washington embraced reports from the United Nations and claims from DR Congo accusing Rwanda and Uganda of providing military support to the M23 in the North Kivu region. Both countries deny this.

The US special envoy for the Great Lakes region, Russ Feingold, a former senator and expert in the region, recalled Wednesday that the sanctions imposed last month were motivated by "the recruitment or assistance in terms of children soldiers for the M23 and involvement of Rwanda in that."

But the punishment could be lifted if it is shown that Kigali has in fact broken ties with the rebels, Feingold said.

A foreign policy priority for Obama

Feingold did not hide the fact that the withdral of Rwandan support for the M23 was due in part to behind the scenes work by American diplomats, in particular to phone calls from Secretary of State John Kerry to Kagame and other leaders in the region.

Without confirming the date of the last conversation between Kerry and Kagame, a State Department official told AFP: "Secretary Kerry and other officials have regularly raised our concerns about support for armed groups such as M23 with various leaders in the region, including President Kagame."

Feingold said the level of US engagement in the region is "probably unprecedented," adding it is one of the first priorities of President Barack Obama's foreign policy.

Feingold noted his own full time appointment to his post in July -- which came at the request of UN counterpart Mary Robinson and EU counterpart Koen Vervaeke -- and the strong involvement of the United States in efforts to conclude regional peace talks underway in Kampala, Uganda for the past 11 months.

After 20 years of war in the region that have left five to six million dead, according to US estimates, Downie agreed with Feingold that "the US is driven really by an humanitarian agenda in the Congo and the genuine desire to try to tackle this conflict."

But, he cautioned, "this a chronic long-running conflict, although we have a moment of optimism right now with M23 disbanding, this is by no means the end of the road in terms of solving these problems."