Syrian warplanes and troops pursued a countrywide offensive, activists and state media have said, bombing rebel-held areas and clashing with armed groups who have pushed into cities.
Government forces clashed on Thursday with rebels in the cities of Deraa, Hama, Homs, Aleppo, Damascus and east of Deir al-Zor, the British-based Syrian Observatory for Human Rights said.
Only the coastal government strongholds of Latakia and Tartous were spared violence.
Opposition activists said 15 people, including 7 children, were killed when an air strike hit a family home in Husseiniyeh, a suburb on the outskirts of the capital. They sent Reuters footage of people dragging the limp bodies of children out of the rubble.
In Hama province, the government said it had secured some areas and displaced families were returning to the area of Zor Abi Zaid after armed forces "cleansed the area completely of terrorists", a term authorities use for the rebels.
Activists and Turkish news agencies reported renewed clashes on the Syrian border town of Ras al-Ain, where rebel forces have been fighting armed Kurdish groups for control.
The Turkish Dogan news agency said one man on the Turkish side of the border was wounded by a stray bullet overnight and that schools in the area had been closed due to the clashes on the Syrian side.
In the power vacuum, some Kurdish groups are trying to assert control over parts of Syria through fights with rebels and government forces. The Observatory said clashes broke out between Kurdish militants and the Syrian army in Rameilan, a town in the northeast.
More than 100 people were shot, stabbed or possibly burned to death by government forces in the Syrian city of Homs, a monitoring group said on Thursday.
The Observatory said women and children were among the 106 people killed by forces loyal to President Bashar al-Assad forces who stormed Basatin al-Hasawiya, a poor district on the edge of Homs on Tuesday.
The town of Houla in Homs province was the scene in May 2011 of the killings of 108 people, including nine children and 34 women, which UN monitors blamed on the army and pro-Assad militia.
The United Nations sent observers to Syria in April 2011 but after several attacks on their convoys they left in August, complaining both sides had chosen the path of war.
The government and opposition blame each other for two explosions at Aleppo's university on Tuesday, which killed at least 87 people, many of them students attending exams.
US ambassador to the United Nations Susan Rice said the attack was "beyond horrific".
"According to eyewitnesses, regime jets launched the strikes," she said on her Twitter account.
Russia, which has backed Assad throughout the revolt both in rhetoric and through its veto of UN Security Council resolutions condemning Assad, dismissed suggestions Damascus was behind the explosions.
"I cannot imagine any bigger blasphemy," Russian Foreign Minister Sergei Lavrov told journalists during a visit to Tajikistan.