The leader of the Democratic Republic of Congo says he is prepared to study the grievances of rebels who have taken the country's main eastern city, Goma. Speaking after talks with his Ugandan and Rwandan counterparts, Joseph Kabila also said he would consider negotiating with the rebels from the M23 movement. But the three leaders called on the rebels to stop their offensive "immediately" and leave Goma. Earlier, the rebels threatened to march on the capital, Kinshasa. On Wednesday they advanced on Sake, 27km (17 miles) west of Goma. They were reported to be preparing to head south to Bukavu, the city about 230km (143 miles) south of Goma. The M23 rebel commander told a crowd in a football stadium in Goma that "the journey to liberate [DR] Congo has started". The BBC's Gabriel Gatehouse in Goma says it is unclear whether the rebels' capacity matches their ambitions, but they currently have momentum on their side. The M23 rebel group was formed in April following an army mutiny. The rebels said they were not given army posts promised in a 2009 peace deal. The M23's gains have raised fears of renewed war in DR Congo, where some five million people died in a conflict from 1997-2003. 'Summary executions' Meanwhile, the UN released a long-awaited report accusing both Rwanda and Uganda of backing the M23, saying the rebel chain of command culminates with Rwandan Defence Minister James Kabarebe. Both Rwanda and Uganda strongly denied the accusations after the report was leaked last month. UN envoy Roger Meece referred to suspicions of international support for the rebels as he briefed the Security Council on Wednesday. "The M23 forces are well provisioned and well supplied with uniforms and a variety of arms and ammunitions, many of which have clearly not come from existing FARDC [army] stocks," he was quoted as saying by AFP news agency. He also cited reports of rebel abuses. "We have received numerous reports of targeted summary executions of those who stand in their way, including government and traditional leaders who resist or fail to co-operate," he said. The rebels entered Goma on Tuesday, skirting round a UN compound as Congolese army forces retreated. France has heavily criticised the failure of the UN force, which has about 19,000 troops, to stop the rebels. On Wednesday the UN defended its actions, saying it had fired hundreds of rockets in an effort to block the rebel advance on Goma. Mr Meece also said UN peacekeepers were stretched, and that the number of rebels advancing on Goma had suddenly jumped from 500 to 3,000 ahead of the fall of the city. UN officials have said they wanted to avoid triggering a battle in Goma, and that they were limited by their mandate. "Clearly, it is not the mandate of Monusco to directly hit the armed groups," said UN peacekeeping chief Herve Ladsous. "They have to be in support of the armed forces of Congo." The UN Security Council has adopted a resolution condemning the rebel seizure of Goma and calling for sanctions against M23 leaders. The rebellion has displaced some 500,000 people since April.