The Syrian government has failed to send a "powerful political signal of peace", UN-Arab League envoy Kofi Annan has told the UN Security Council.
The Syrian army continued operations in population centres despite an agreement to pull out troops by Tuesday, he said.
Syrian opposition activists said at least 60 people were killed as the army shelled a number of towns.
Despite the clashes, Mr Annan said his peace plan was still alive as he urged both sides to stop fighting.
"The days before 10 April should have been an opportunity for the government of Syria to send a powerful political signal of peace, with action on all aspects of the six-point plan," he wrote in a report to the Security Council.
"I am gravely concerned at the course of events."
'Give no excuse'
Under his plan, sponsored by the UN and the Arab League, Syrian troops were to have completed their withdrawal from population centres and stopped the use of heavy weaponry by Tuesday, ahead of a full ceasefire coming into place on Thursday morning.
Damascus had agreed to the deadline, but on Sunday demanded written guarantees first that its opponents would give up arms, along with a promise from foreign states not to fund them.
Mr Annan told the Security Council that this new demand put the cessation of violence at risk.
Shelling was reported in several cities on Tuesday, including Homs, a centre of resistance to the rule of Syrian President Bashar al-Assad since the rebellion broke out in March last year.
At least 28 people were killed there, said the Local Co-ordination Committees (LCC), an activist network.
The LCC said 20 people were killed in Hama, while there were further deaths in Aleppo and Idlib in the north, Deraa in the south and in a Damascus suburb.
Mr Annan said there was still a chance to salvage the peace plan.
"It is essential that the next 48 hours bring visible signs of immediate and indisputable change in the military posture of the government forces throughout the country," he told the Security Council.
He added that the opposition should also stop fighting in order to "give no excuse for the government to renege on its commitments".
The Security Council was in agreement with Kofi Annan that the Thursday deadline must be adhered to, said the US ambassador to the UN, Susan Rice - who is president of the council this month.
"Members of the council are unified in their grave concern that this deadline has passed and the violence has not only continued but over the last 10 days has intensified," she said.
She said the council faced a "moment of truth coming up" when it would have to decide whether to increase pressure on the Syrian government.
Permanent members Russia and China, who hold vetoes over council decisions, have prevented resolutions condemning Damascus but have backed Mr Annan's peace plan.
Russia is one of Syria's closest allies.
Speaking earlier in Turkey, after visiting a camp for Syrian refugees, Mr Annan said it was too early to say his plan had failed.
"It is a plan the [UN Security] Council has endorsed. It's a plan the Syrians have endorsed and from the comments made by the opposition, they are also prepared to go along with it if the
government meets its commitments to pull the troops out. So I think the plan is very much alive."
Speaking in Moscow, Syrian Foreign Minister Walid Muallem said the withdrawal had begun in some areas, but that the ceasefire should be timed to the arrival of international monitors - something Mr Annan said he was working towards.
But Mr Muallem said that "despite all these positive measures we noticed on a daily basis the escalation of opposition by the armed terrorist gangs".
Syrian opposition representatives have said they are committed to the peace plan but that if government troops did not stop firing by Thursday, they would intensify their own operations.
The Russian foreign ministry said Foreign Minister Sergei Lavrov had urged Mr Annan in a t
elephone call to put pressure on countries backing the Syrian opposition to make sure they stuck to the ceasefire plan.
Earlier he said the Syrian government "could have been more active and decisive" in implementing the plan, but that Mr Muallem had assured him Damascus was committed to it.