The Socialist EDEK party quit Greek Cyprus's left-wing coalition government on Monday, accusing Christofias of making unacceptable concessions in peace talks with Mehmet Ali Talat, the Turkish Republic of Northern Cyprus president.
The talks, seeking to end decades of division and a conflict, started in September 2008.
"We are not so close to a solution as some would suggest both here and abroad, but we are working for a solution and this process is ongoing," Christofias said in a statement.
"No one can pre-judge the outcome, and certainly not suggest that we are trapped on a dangerous path. That is unsubstantiated and wrong," Christofias said.
EDEK and the centrist Democratic Party had supported Christofias, a communist party leader, in presidential elections in 2008.
Their departure does not mean early elections, since Greek Cyprus has a presidential system of government with a strong executive. It does however underline the many obstacles both Greek and Turkish Cypriots will need to overcome before coming anywhere close to a deal.
Talat, Christofias's key negotiating partner in talks, is also not without domestic problems. He faces elections in April and could lose to a hardliner.
EDEK, seen hawkish in their approach to reunification talks, have repeatedly said Christofias's decision to start negotiations failed to meet a condition that there be a common basis for discussions. The Democratic Party shares EDEK's misgivings.
Greek and Turkish Cypriots are attempting to reunite Cyprus as a two-zone federation, ending an ethnic split since a Turkish intervention in 1974 after a brief Greek-inspired coup.
Grievances are focused on Christofias's offer of a rotating presidency with Turkish Cypriots and the voting system.
"The basic principle is that nothing is agreed until everything is agreed," Christofias said. "The problems (in the negotiation process) are tangible, as is the prospect of solving the Cyprus problem."
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