M23 rebels withdraw from the city of Goma in the eastern Democratic Republic of Congo on December 1, 2012.
A UN report says Rwanda and Uganda helped rebels in the Democratic Republic of Congo launch a major offensive in the east of the country last month.
The report by UN sanctions experts said that hundreds of Rwandan troops bolstered M23 rebels as they took the key Congo’s eastern city of Goma, while Uganda provided "logistics" support, AFP reported on Monday.
The March 23 movement (M23) rebels seized Goma on November 20 after United Nations peacekeepers gave up the battle for the frontier city of one million people, which is the capital of North Kivu province. The rebels withdrew from the city at the weekend under a ceasefire accord.
The experts said that their investigation "strongly upholds" previous accusations that the neighboring countries are backing the M23.
Neither country immediately commented on the new report, which has been sent to the UN Security Council for consideration.
The Security Council has already issued sanctions against three M23 military commanders. Last week the council passed a resolution, which threatened action against "those providing external support" to the rebels.
In October, the United Nations Security Council’s Group of Experts said in a confidential report that Rwanda and Uganda continued to support the March 23 movement, which is strengthening its grip over the provinces of North Kivu and South Kivu in eastern Congo and could seriously threaten the writ of the Congolese government in the region.
Both Uganda and Rwanda strongly denied the UN accusations.
The M23 rebels defected from the Congolese army in April in protest over alleged mistreatment in the Armed Forces of the Democratic Republic of Congo (FARDC). They had previously been integrated into the Congolese army under a peace deal signed in 2009.
Since early May, over 750,000 people have fled their homes in the eastern Congo. Most of them have resettled inside Congo, but tens of thousands have crossed into neighboring Rwanda and Uganda.
Congo has faced numerous problems over the past few decades, such as grinding poverty, crumbling infrastructure, and a war in the east of the country that has dragged on for over a decade and left over 5.5 million people dead.