Yesterday it was the turn of Belgium's Foreign Minister, Erik Derycke, to renounce publicly an old ally. Speaking in Brussels he said that Belgium - the colonial power in the former Congo - believed that Mr Mobutu had no future as leader of Zaire.
"It is military dictatorship revisited, with a military man as prime minister and the parliament thrown aside," he said.
Belgium's announcement echoed statements on Wednesday by the White House spokesman, Mike McCurry, and US State Department official Nicholas Burns.
"We have suggested that the era of Mobutism in Zaire is over because the status quo is no longer tenable, given the dire conditions that exist for the people of Zaire," Mr McCurry said in Washington. "It's clear we have to move beyond Mr Mobutu."
France, the other member of the troika of Western powers most involved with Zaire, has yet to respond to the US and Belgian statements. Historically the most closely involved with Mobutu, France has offered Mr Mobutu diplomatic support against Laurent Kabila's six month old rebellion but has failed to intervene directly to save him, as it did on several occasions in the past.
Mr Mobutu seems to be backing himself into a military cul de sac. On Wednesday Mr Kabila's rebels over-ran Lubumbashi, Zaire's second city. The same day in the capital, Kinshasa, government troops used tear gas to disperse opposition students demonstrating in defiance of the state of emergency declared by Mr Mobutu on Tuesday night.
The veteran opposition leader and prime minister-designate, Etienne Tshisekedi, was arrested and later released, but only after Mr Mobutu announced that he was using emergency powers to sack him and appoint a former defence minister, General Likulia Bolongo, as his new premier.
US diplomats have long privately acknowledged that, with the Cold War over, America no longer has any need to shore up Mr Mobutu's brutal and staggeringly corrupt regime. But with Mr Mobutu now abandoning even the pretence of civilian government, the US has felt it necessary to drive this point publicly home.
During the Cold War years Mr Mobutu became the West's principle ally in Africa. US financial and diplomatic support, together with military intervention from France and Belgium, rescued Mr Mobutu from several uprisings. In return, Mr Mobutu provided supply bases for the US- and South Africa- backed Unita rebel movement in Angola.
For nearly three decades Mr Mobutu's Western allies were content to ignore his numerous faults. In what came to be known as a system of kleptocracy - "rule by thieves" - Mr Mobutu and his cronies plundered Zaire's huge mineral wealth, squirrelling billions of dollars away in Swiss bank accounts. Meanwhile the colonial infrastructure collapsed.
Another reason for the US change of heart on Mr Mobutu is the growing alliance between Washington and the central African states of Rwanda and Uganda, which are closely linked to Mr Kabila's Alliance of Democratic Forces for the Liberation of Congo-Zaire.
It is understood that the US has been using intermediaries such as Morocco to try and persuade the ailing Mr Mobutu that, after 32 years in power, it is time to give up.
The capture of copper and cobalt rich Lubumbashi comes less than a week after rebels took the diamond capital of Mbuji-Mayi and gives Laurent Kabila effective control over Zaire's mineral wealth. Diplomats say that, having cut off Mr Mobutu's funds, Mr Kabila can now sit back and wait for the the regime to fall apart.
Yesterday, however, Mr Kabila vowed to continue the military advance until Mr Mobutu was deposed. Speaking in Goma on Wednesday, Mr Kabila said his forces would give the President three days to begin negotiating a handover of power, following which military action would be taken.
"He can do it," Mr Kabila said. "He is tired and worn out, but he is also scared. He has to be helped to overcome this fear."