Taylor, a former warlord blamed for 14 years of conflict in Liberia, has said since early June he would yield power, as the Bush administration has demanded.
Taylor has hedged or reneged outright on the promise since, but most recently said he would step down when long-promised multinational peace forces arrived.
Now Taylor sees his promise as only encouraging rebel attacks, spokesman Vaanii Paasawe said.
In neighboring Ghana, meanwhile, the country's main rebel group, which has staged a 10-day siege of Monrovia, declared the latest in days of cease-fire pledges. But it stopped short of U.S. demands that they withdraw and open the city's port for vital deliveries of food and other aid.
Taylor's government reacted with skepticism. Like Taylor's forces, the rebels repeatedly have broken promises for a cease-fire.
Meanwhile, military officials, speaking on condition of anonymity, reported a "big fight" at the Atlantic coastal city of Buchanan.
Monday's attack on Buchanan marked a return to full-scale fighting by the country's smaller rebel group, Movement for Democracy in Liberia.
Unlike the larger Liberians United group, the Movement for Democracy largely had heeded cease-fire pledges made at off-and-on peace talks in Accra, Ghana.
Aid workers, tending to malnourished children in the capital, decried the loss of Buchanan, which had been the last significant port in government hands.
"Buchanan was the only alternative way to ship some food into Liberia. Now — you can forget about it," Frederic Bardou said at a feeding center in Monrovia run by Action Contre la Faim, or Action Against Hunger.
Around him at the center, emaciated babies hung from their mothers' shoulders.
Rebels also hold Monrovia's port, cutting off warehouses filled with food from the city's increasingly hungry and disease-ridden populace of more than 1.3 million.
Officials said Taylor's forces also were struggling to take back Taylor's northern stronghold of Gbarnga, which like Buchanan fell to insurgents on Monday.
Government forces had retaken a military base, Naama, to the north of Gbarnga, a commander said.
Rebels have waged a three-year campaign to oust Taylor, who was elected president in Liberia in 1997 after waging eight years of civil war — a conflict that spread into neighboring Sierra Leone. He has been indicted by a war crimes court in Sierra Leone for backing that country's brutal rebels.
In Accra, West African mediators convened envoys of Liberia's government and the two rebel groups to urge all sides to abide by a repeatedly flouted June 17 truce.
"We are asking them to cease-fire immediately," said Sunny Ugoh, spokesman of the leading West African regional bloc. "This will not be negotiable."
West African leaders have promised a multinational peace force for Liberia since soon after rebels launched their siege of Monrovia in early June. None has deployed yet.
More West African, U.N. and U.S. talks on the peace force ended Monday with no sign of progress on deployment.
Debt-strapped Nigeria, West Africa's military power, has offered to send at least two battalions but says it needs help from the United States and others to foot the bill.
The United States, which oversaw Liberia's founding by freed American slaves in the 19th century, has pledged support but insists Liberia's neighbors and the United Nations must take the lead.
President Bush on Friday ordered troops to take up position off Liberia's Atlantic coast in readiness for any peace mission.
The Bush administration has insisted no U.S. troops will become involved until Taylor leaves and a cease-fire takes hold. The president has voiced worries about over-extending a military already committed to Afghanistan and Iraq. The last major U.S. peacekeeping mission in Africa, in Somalia in 1993, claimed 18 American lives.