Monitors said 31 people were killed on Sunday, a day after nearly 130 died across Syria, while 200 others were arrested in raids by government forces pushing to crush dissent.
"To say that Syria will pull back its forces from towns on April 10 is inaccurate, Kofi Annan having not yet presented written guarantees on the acceptance by armed terrorist groups of a halt to all violence," the foreign ministry said in a statement.
On Thursday, the UN Security Council formally endorsed the deadline, but Damascus said a day later that the number of "terrorist acts" has risen since the deal was agreed with the UN and Arab League envoy.
"Mr Annan has not submitted written guarantees from the governments of Qatar, Saudi Arabia and Turkey on stopping their funding to terrorist groups," the foreign ministry said of its key regional critics.
Annan's peace plan calls for the withdrawal of the Syrian army from protest cities on Tuesday, with a complete end to fighting 48 hours later.
"Syria is not going to repeat what happened in the presence of Arab observers when armed forces left towns," the foreign ministry said.
"Armed terrorist groups reorganised and rearmed to control entire neighbourhoods, committing every possible terrorist act, killing and kidnapping people and destroying public and private property."
Annan "said he would work to stop the violence, disarm armed groups... initiate a comprehensive national dialogue with opposition movements," when he met President Bashar al-Assad last month, the ministry said.
"It is this principle on which Syria accepted Annan's mission and his six-point plan."
In a statement in Geneva, Annan expressed "shock" at the surge in violence, calling the bloodshed "unacceptable" and urging the government in Damascus to respect its commitments.
"I remind the Syrian government of the need for full implementation of its commitments and stress that the present escalation of violence is unacceptable."
"This is a time when we must all urgently work towards a full cessation of hostilities, providing the space for humanitarian access and creating the conditions for a political process," Annan said.
The former UN chief said he was in "constant contact" with the government, and asked "all states with influence on the parties to use it now to ensure an end to the bloodshed and the beginning of dialogue."
Syrian Foreign Minister Walid Muallem is scheduled to travel to ally Moscow on Monday.
The United Nations says more than 9,000 people have been killed since anti-regime protests broke out in March 2011, while monitors put the number at more than 10,000.
The main umbrella opposition group the Syrian National Council called for UN intervention after monitoring groups said 86 of nearly 130 people killed on Saturday were civilians.
Another 31 people were killed across the country on Sunday, including 12 civilians, monitors said.
"We urge the UN Security Council to intervene urgently to stop the humanitarian crisis caused by the regime against the unarmed Syrian people, by adopting a resolution under Chapter 7 to ensure the protection of civilians," the SNC said.
"The regime has committed savage massacres that killed nearly 1,000 people since it falsely announced the acceptance of the (Annan) plan."
The SNC and the rebel Free Syrian Army "have announced their cooperation with the Annan plan" the group said, as monitors reported a series of clashes between rebels and government troops on Sunday.
Pope Benedict XVI called in an Easter Day message for an end to the killings in Syria and a commitment to dialogue, as part of a wider appeal to give hope to the entire Middle East.
"Particularly in Syria, may there be an end to bloodshed and an immediate commitment to the path of respect, dialogue and reconciliation, as called for by the international community," he said in his traditional "Urbi et Orbi" message.