US Secretary of State Antony Blinken said on Tuesday Washington was "very concerned" about what he called credible reports that Rwanda has provided support to M23 rebels in the neighbouring Democratic Republic of Congo.

Blinken, speaking during a visit to Kinshasa ahead of a trip to Rwanda, called on all parties to halt any support for or cooperation with M23 or other non-state armed groups.

Blinken said the conflict in eastern Congo was a focus of his meeting with Congolese President Felix Tshisekedi on Tuesday and would be central when he meets Rwandan President Paul Kagame on Wednesday.

The M23 insurgency is part of the fallout from the 1994 genocide in Rwanda. The group was formed in 2012 claiming to defend Congolese Tutsis, Kagame's ethnic group, against Hutu militias.

Since May, M23 has waged its most sustained offensive in years, killing dozens and displacing tens of thousands of people. By July, it controlled a territory in Congo almost three times as large as it did in March, a UN group of experts said.

Rwanda has previously denied accusations by Congo's government that it supports M23 and that it has sent troops into the country. M23 has denied it receives Rwandan support.

During a joint press conference with Congo's Foreign Minister Christophe Lutundula, Blinken said he also discussed reforms to Congo's mining sector and US concerns over the auction of oil and gas exploration blocks close to sensitive rainforest and wetland areas.

Congo on Tuesday agreed to establish a working group with the United States to discuss the environmental impact of the auctions.

Lutundula said regarding the oil and gas blocks that Congo had to find a balance between the need to support its people and economy and its commitment to protecting the environment.

"We stand firm on this commitment," he said.