The head of the UN, Ban Ki-moon, has condemned the Syrian government for fresh military assaults despite agreeing to a deal to end the violence.
Mr Ban said the 10 April ceasefire deadline was "not an excuse for continued killing" by forces loyal to President Bashar al-Assad.
Activists say at least 100 people have been killed over the past two days as troops intensified operations.
Earlier, Turkey said it may need UN help after a surge in refugee arrivals.
After speaking to Mr Ban, Foreign Minister Ahmet Davutoglu said the rate of refugees had doubled since Mr Assad agreed to implementing the UN-Arab League plan.
The six-point peace plan, mediated by envoy and former UN chief Kofi Annan, envisages cessation of armed violence by all parties from 10 April, with a full ceasefire on 12 April.
However, the Syrian opposition, the US and many regional countries have expressed scepticism that Syria is committed to the plan.
In Friday's statement, Mr Ban said the recent attacks on civilians "violate" the UN's demands and demanded the government halt military operations.
Mr Ban "deplores the assault by the Syrian authorities against innocent civilians, including women and children, despite the commitments by the Government of Syria to cease all use of heavy weapons in population centers," it said.
"The 10 April timeline to fulfill the Government's implementation of its (ceasefire) commitments, as endorsed by the Security Council, is not an excuse for continued killing," it said.
Earlier, Turkey said that if the influx of refugees continued at the current rate, it would need international assistance. More than 2,800 Syrians have crossed over the border in 36 hours, with the total now near 24,000, Mr Davutoglu said.
Many of the refugees have reported intense bombardment by government forces.
On Friday, activists said violence was continuing across the country.
The BBC's Jim Muir in Beirut says that in Homs and some other areas the approach of the deadline to halt all violence seems to have brought an escalation rather than a reduction.
He says activists are accusing the government of trying to complete its crackdown come what may; but government officials say it is the rebel fighters who are exploiting the impending withdrawal of the military from towns and cities.
A UN team is currently in Damascus to negotiate the possibility of deploying UN monitors to oversee any ceasefire.
Mr Annan has said that if the truce is successful a small mobile UN monitoring mission of some 200-250 observers could be brought into Syria.
The UN estimates more than 9,000 people have died in the year-long uprising against the rule of President Assad.