Separately, the government of Laurent Kabila accused the rebels, who are backed by Uganda and Rwanda, of using US mercenaries in their campaign against the Congolese president.
From Kigali, Rwanda, rebels called on President Kabila to halt the bombardment of Bokungu, 500 miles east of the Congolese capital, Kinshasa. The attack, allegedly involving Antonov aircraft, helicopter gunships and heavy artillery, seemed to be aimed at breaking through a rebel line to link up with Ikela, where the 700 Zimbabweans have been surrounded since at least Tuesday.
The Democratic Republic of Congo, which is almost the size of Western Europe and used to be known as Zaire, has been at war since August last year. President Kabila, who overthrew Mobutu Sese Seko in 1997, is backed by Zimbabwe, Namibia and Angola. The rebels, in a power challenge with ethnic overtones, argue that he has done nothing to move the country towards democracy.
In the past year, the rebels have become divided by Rwanda's and Uganda's different motivations for backing them. Kin-Kiey Mulumba, a rebel spokesman speaking in Kigali, said of the Bokungu bombings: "The fighting is extremely violent. We are continuing to resist. But if they continue, the Zimbabweans at Ikela will find themselves in a bloodbath."
Mr Mulumba said the fighting was the fiercest since the peace accord signed in August in the Zambian capital, Lusaka, which aimed to establish a ceasefire and pave the way for peace-keepers.
Earlier this week in New York, Namibia led the Security Council in urging the UN to arm 500 military observers for Congo soon - as forerunners of the 25,000 peace-keepers promised under the Lusaka deal. There are already observers in the Congo, but with limited remit and resources. Further difficulties have been caused by the flooding that has affected 15 of Kinshasa's 24 districts and made about 10,000 people homeless.
The Namibians' Security Council resolution was mainly an attempt at shoring up the Lusaka agreement against frequent claims, from all parties in the conflict, of breaches of the ceasefire. The US Congress has yet to consider the cost of the larger peace-keeping initiative. And the UN is wary of deploying peace-keepers when there is no peace to keep.
It is understood that attempts to secure a safe passage for the Zimbabwean forces trapped in Ikela, nearly 200 miles east of Bokungu, failed when the soldiers refused to surrender their weapons.
Asked about the Congolese and Zimbabwean claims that US mercenaries were manning rebel artillery and communications equipment in Lusambo and Kabalo, in central Congo, Mr Mulumba said: "I am categorically denying that we have any mercenaries on our side. Least of all, Americans."
But Zimbabwe's defence forces spokesman, Colonel Chancellor Diye, said: "The presence of white mercenaries from the United States has been noticed. About 30 were seen at Lusambo and Kabalo."
Colonel Diye did not say whether the mercenaries had the support of the American government; nor did he name the organisation they represented. He said violations of the ceasefire by the rebels had continued unabated despite the deployment of monitors from the Organisation of African Unity and the UN in the past two weeks.
President Robert Mugabe of Zimbabwe has assigned about 11,000 troops - one-third of the country's army - to support Mr Kabila, a deeply unpopular move at home, where the economy is in crisis. The International Monetary Fund, the World Bank and other donors have suspended aid programmes to Zimbabwe because of differences on economic management, especially government spending in the Congo.
Zimbabwe's Foreign Minister, Stanislaus Mudenge, said the rebels had moved reinforcements and heavy weapons near areas held by the pro-Kabila allied forces, in preparation for a big offensive. He said the violations of the ceasefire would have a negative impact on the peace process.
Colonel Diye said the allies would defend themselves "to the teeth" if attacked.Reuse content