Senior Western diplomats in Africa believe that Mr Kabila's troops are capable of taking the town, Zaire's third-largest city and headquarters of the government's flagging counter-offensive. "They'll probably take the city by the end of this month," one said. Strategically located at the headwaters of the country's most significant artery, the River Zaire, Kisangani is viewed as the key to controlling the sprawling interior.
Zairean defence officials said that aircraft which bombed the towns of Bukavu, Shabunda and Walikale on Monday took off from Kisangani. Aid officials said that at least nine people were killed and up to 37 wounded in Bukavu, and that thousands of refugees were fleeing.
Mr Kabila yesterday used an interview on French television to issue his threat. "These are terrorist actions," he said of the bombing. "And we are going to get ready to take the war precisely to the place from where the bombers took off."
A senior defence official said that Goma, the biggest city in rebel hands, was the next target. Bombing raids on nearby Bukavu and other parts of eastern Zaire have reignited fears of a concerted government campaign against civilian targets. But among Goma's inhabitants, belief in the rebels' ability to win the war has not been shaken. Billed last month as "total and devastating", the government's counter-offensive has failed to halt the rebel advance.
The fighting is now taking place far to the west, deep in the equatorial rainforest. Since launching their campaign last October, the rebels have taken control of a huge swath of eastern Zaire and renamed it the Democratic Republic of Congo. Not a week goes past without the capture of a new town or strategic junction.
Led by the long-time revolutionary Mr Kabila, the Alliance of Democratic Forces for the Liberation of Congo-Zaire (ADFL) has enjoyed astounding success. With the exception of a couple of initial battles, the Zairean army (FAZ) has hardly bothered to engage the rebels. Instead, the demoralised troops have retreated, raping and looting on their way.
"It is not so much that the rebels have succeeded," a Western diplomat in the region said. "It's more a question of the FAZ having failed. They're without logistics, training and a will to fight. Even the mercenaries hired by the government have taken a beating."
Suggestions that the rebels' success could only have been achieved with the backing of Rwanda and Uganda continue to be denied by the ADFL and by the country's eastern neighbours. Western diplomats in the Rwandan capital, Kigali, insist that there is no evidence of direct involvement by either country. Some, however, concede that Rwanda might be providing the rebels with military instructors. Sources inside Zaire indicate that the ADFL's officer cadre is being trained at a camp inside Uganda. "We are doing this on our own," the ADFL commissioner of information, Raphael Ghenda, said, seated in his office in central Goma. "The arms we have were left behind by the fleeing Zairean army. I can't say where we bought our uniforms. But we have used the riches of the region to support our struggle."
So brazen has the rebel thrust become that Mr Kabila has given President Mobutu Sese Seko until this Friday to resign. If he does not, the rebel leader says he will march his men right across the country and into the capital, Kinshasa.
"There's quite a lot of propaganda with this regime," an aid worker in Goma said. "But, so far, everything they've said has come true. If they say they're going to take a town, they take it. In fact, that's part of their strategy - to announce their next step and wait for the FAZ to flee."
A new offensive against Kisangani could have important results. "The effect on the government of the rebels taking Kisangani would be devastating," a high-ranking Western diplomat in the region said. "The negotiations will start if they reach Kisangani." What shape talks might take is difficult to envisage. Despite his precarious state of health after a cancer operation, President Mobutu, 66, has vowed to crush the rebels by force. For his part, Mr Kabila insists that no negotiations can begin until the President stands down.
Anxious to avert a costly humanitarian crisis in Zaire - the almost inevitable fall-out of prolonged conflict in Africa's third largest country - the United States has been actively engaged in mediating between the two sides. One scenario is that President Mobutu, an old Cold War ally, might be prevailed upon to take his plundered fortune and settle quietly in exile. This would leave the way open for Mr Kabila to make a deal with the legitimate opposition and form an interim government. On one point most observers are in agreement: President Mobutu's star is in decline.