The Democratic Republic of Congo government has rejected calls for talks with a rebel leader whose forces control eastern areas, reported BBC.
Gen Laurent Nkunda has said his forces would topple the government if no agreement to negotiate is struck.
But DR Congo's ambassador to the United Nations said Gen Nkunda should be in jail for war crimes.
Gen Nkunda commands 6,000 Tutsi rebels in the east, where some 250,000 people have been displaced by recent fighting.
The rebel leader says his forces are respecting a ceasefire, although they still surround the city of Goma, which is protected by UN peacekeepers.
In an interview with the BBC, Gen Nkunda reiterated a threat to overthrow the government of President Joseph Kabila unless it holds talks.
However, correspondents say it is not clear that his small force could really threaten the capital, Kinshasa, 1,600km (1,000 miles) to the west.
Gen Nkunda's remarks were summarily dismissed by Atoki Ileka, the country's UN envoy, who called the rebel leader "a killer".
"Laurent Nkunda has been labelled by the United Nations since 2002 as a warlord and a war criminal since the massacres in Kisingani in 2002 when he was general for the [rebel] RCD Goma," Mr Ileka said.
"So I don't have any trust in him. What I want - like I said to him about four years ago - [is] that one day I'm going to visit him. I'm going to visit him when he's going to be in jail.
"You don't negotiate with criminals, you catch a criminal and put him in jail so he can respond to justice."
But Gen Nkunda said the way to resolve the current crisis was through negotiation.
"If he [President Joseph Kabila] refuses to talk now he will not be able to rule Congo and to lead it," he said.
"We have to liberate Congo. Congo has to be free and to develop."
Gen Nkunda has always said he is fighting to protect his Tutsi community from attacks by Rwandan Hutu rebels, who fled to DR Congo after Rwanda's 1994 genocide.
He spoke to the BBC at his mountain headquarters, three hours' drive outside Goma.
But he says it raises the stakes at an extremely tense time, when other African countries are also threatening to intervene on the side of the elected Congolese government.
In the most recent clashes, Gen Nkunda's forces have taken a series of towns and villages near Goma, the capital of DR Congo's North Kivu province.
The UN has accused both sides of war crimes, following the reported killing of several civilians in the eastern town of Kiwanja last week.
Gen Nkunda claimed his forces had simply been responding to attacks.
"Since we declared the ceasefire, we were attacked around six times or seven times," he said.
"We are reacting only, but the ceasefire is there."
On Sunday, Southern African leaders pledged to provide military advice to the Congolese army and said they would send a peacekeeping force if and when necessary.
Rwanda, which is led by Tutsi President Paul Kagame, has denied claims that it has been backing Gen Nkunda.
The UN has 17,000 peacekeepers in DR Congo, its largest mission in the world, but only a few hundred peacekeepers are in the areas affected by the latest violence.