The Syrian National Council said on Tuesday that the Damascus regime had so far failed to comply with a UN-backed peace plan and described new conditions it set as "unacceptable and unrealistic".
The use of heavy weaponry and mortar shelling has been reported on Tuesday, the day the plan had called for Syrian troops to pull out of rebel cities, SNC spokeswoman Basma Qoudmani said on a visit to Geneva.
The SNC said 160 lives were lost alone on the eve of the plan's implementation alone.
"I think we have received a very strong message from the regime that it intends to intensify rather than beginning to comply with the ceasefire," Qoudmani told reporters.
"From our perspective and based on our information there is no compliance at all for the moment."
The UN Security Council formally backed an April 10 deadline envoy Kofi Annan agreed with Damascus to draw back its troops and armour from population centres on Tuesday ahead of a complete ceasefire on Thursday.
Damascus said on Sunday it would not withdraw forces without "written guarantees" that the opposition will also lay down arms.
"Clearly if they had thought about the statement a little bit they would have realised that getting any written committment by any group... they would have to recognise these groups and work with them, so I think they backed off on this one," said Qoudmani.
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"The second unrealistic demand is when they say that monitors or observers should be on the ground before a ceasefire.
"There is no international peacekeeping mission that ever occurred without a ceasefire beginning to be implemented.
"They are there to monitor or observe the implementation of the ceasefire, otherwise they are troops that are asked to fight."
Qoudmani said the SNC would fully cooperate with the Annan mission and that groups on the ground were committed to the ceasefire.
Any use of arms by the Free Syrian Army or by civilians has been done so in self-defence, said the spokeswoman, "therefore it is not an issue or a dilemma in any way to say they can commit to cease using these arms if there is no need for self defence."
"Today the onus lies with the Syrian regime."
The SNC said heavy gunfire from security checkpoints has been reported on Tuesday in Daraa, the cradle of the dissent movement and tanks have been deployed in many cities including Douma which lies near the capital.
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.
"We continue to consult with the Annan mission and seek some form, any form of compliance, or any means to get the regime to comply with the ceasefire," Qoudmani said.