A military bombardment of a town in central Syria killed 30 people on Monday on the eve of a scheduled army withdrawal from urban areas, opposition activists said, dashing the prospects of a UN-brokered ceasefire taking hold.
Government troops and rebel forces also clashed near Syria's border with Turkey, activists said.
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 unrelenting violence indicated that a peace plan promoted by international envoy Kofi Annan and initially accepted by both sides was in tatters.
Syria was to have started pulling troops out of towns and cities by Tuesday, paving the way for a ceasefire to start 48 hours later.
But President Bashar al-Assad on Sunday 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 on Monday.
"April 10 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 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 Annan's efforts.
Russia, which has defended him in the UN Security Council and remains 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."
WOMEN AND CHILDREN VICTIMS
Opposition activists said the military bombarded al-Latmana, northwest of the city of Hama, killing 30 people. Among the dead were 17 children and eight women, they said. Hama has been in the forefront of the uprising which began with street protests in March 2011 and has since intensified into armed conflict.
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.
The state news agency SANA said nine security men and a civilian were shot dead and 13 wounded in Aleppo, Syria's second city. The Observatory said two policemen were killed in clashes with gunmen in Aleppo, a commercial center that has generally been pro-Assad.
In Damascus province, four soldiers were killed in the bombing of a convoy as Syrian forces swept villages arresting opposition suspects, the Observatory said.
Security forces on Sunday foiled infiltration attempts of two armed groups from Lebanon into the Homs area, SANA said. Bloodshed also spilled into northern Lebanon, where a cameraman for Al-Jadeed television channel was shot dead. Sources at the channel said the shooting occurred in the border region between Syria and Lebanon's northern Wadi Khaled district.
Colonel Qassem Saad al-Deen, a spokesman for the Free Syrian Army, said on Sunday that at least 1,000 people had been killed during the past week, most of them civilians.
Reports from both sides are difficult to verify independently as the government restricts access for most foreign reporters. Anti-Assad activists say the army is trying to seize back swathes of northwestern Idlib province from rebel control, using tanks and helicopters and driving terrified villagers north and west to the Turkish frontier.
Syria blames the uprising on foreign-backed terrorists determined to destabilise the government.
Assad, who has ruled for 10 years since succeeding his late father, Hafez al-Assad, has laid out his own reform programme but it has been dismissed by the opposition.
The United Nations says government forces have killed 9,000 people in the crackdown, while the government says more than 3,000 soldiers and security personnel have been killed by rebels.