A military bombardment of a town in central Syria killed 30 people today on the eve of a scheduled army withdrawal from urban areas, opposition activists said.
Government troops and rebel forces also clashed near Syria's border with Turkey, according to reports.
Two Syrian refugees and a Turkish translator were wounded by gunfire from Syria at a refugee camp in Turkish territory, Turkish officials said, drawing an angry response from the Ankara government.
The violence indicated that a peace plan promoted by international envoy Kofi Annan and initially accepted by both sides is unlikely to be observed.
Syria was to have started pulling troops out of towns and cities by 6am tomorrow morning, paving the way for a ceasefire to start 48 hours later.
But President Bashar al-Assad yesterday said his foes must give written guarantees they would stop fighting and lay down their arms - a demand they immediately rejected.
Nor did government forces show any sign they were starting to pull back today.
"10 April has become void," Turkish Deputy Foreign Minister Naci Koru said in Ankara, referring to the deadline.
European Union foreign policy chief Catherine Ashton said in Brussels that adding new conditions was totally unacceptable.
China, which has supported Mr Assad in his year-long effort to crush the uprising against his family's dynastic rule, called on both sides to honour the ceasefire and support Mr Annan's efforts.
Russia, which has defended him in the UN Security Council and remains Mr Assad's most important ally, stopped short of pressing him to rein in his army.
"Attempts to force a solution on Syria from outside will lead only to an escalation of tension," Russian Deputy Foreign Minister Gennady Gatilov said. "Everything must follow from respect for Syria's sovereignty, and violence must be stopped."
Middle East specialist Augustus Richard Norton of Boston University said the collapse of the ceasefire was unsurprising.
"The Syrian regime does not understand compromise. Its ethos is 'rule or die," he told Reuters in Beirut.
"Therefore, Syria will continue its inexorable slide into full-scale civil war, especially since the chance for effective foreign intervention to stop the bloodletting is also zero."
Opposition activists said the military bombarded al-Latmana, northwest of the city of Hama, killing 30 people.
Hama has been in the forefront of the uprising which began with street protests in March 2011 and has since intensified into armed conflict.
Reports from Syria are difficult to verify as the government has restricted access by foreign journalists.
In clashes near the Turkish border, rebels killed at least six members of the Syrian security forces and customs officials, according to the opposition Syrian Observatory for Human Rights.
Eight rebel fighters were wounded in the action, which took place in Salama village, between the Syrian town of Azaz and the Turkish town of Kilis, the British-based Observatory said.