Syrian opposition forces said there were almost immediate violations of a unilateral three-day truce declared on Wednesday by President Bashar al-Assad's government.
The rebels called the ceasefire announcement, reported by state-run news agency SANA, nothing more than "propaganda" as media outlets in rebel-held Aleppo reported injuries from government shelling.
The Britain-based Syrian Observatory for Human Rights confirmed breaches of the truce within hours of it being announced, with government air raids on the outskirts of the northern city of Aleppo.
The Army of Islam rebel group said that regime forces launched an attack on the town of Maydaa on the outskirts of Damascus. The regime had already captured most of the town in ongoing fighting, according to the Observatory.
The "regime of calm," as the government referred to the three-day truce, had already entered into effect and would last until midnight on Friday, according to the SANA report.
The ceasefire coincides with the Eid al-Fitr holiday marking the end of Islam's holy month of Ramadan.
In a rare public appearance outside Damascus, al-Assad participated in holiday prayers at a mosque in Homs, a city in central Syria which has seen heavy battles during the civil war but is now under government control.
State television broadcast pictures of al-Assad at the al-Safa mosque, in an area which has been frequently targeted in suicide attacks blamed on Islamic State and al-Qaeda's Syrian wing, al-Nusra Front.
The Observatory meanwhile said that the death toll in a Tuesday evening bomb attack on a bakery in the north-eastern city of al-Hassakeh, mostly controlled by Kurdish forces, had risen to 25.
Local authorities cancelled Eid celebrations in the Kurdish-governed region of northern Syria after the attack. Islamic State supporters on social media circulated a claim of responsibility for the attack, which dpa could not verify.
In February, after nearly five years of conflict in Syria, the United States and Russia brokered a nationwide ceasefire that held for a time but then collapsed under the pressure of localized fighting and a failed UN-backed political process to end the violence.
There have been attempts since then to reach short-term, often region-specific, cessations of hostilities, but these have tended to be short-lived.
The Observatory said that 1,138 Syrian civilians, including 249 children, were killed during Ramadan, which started on June 6.
Of these, 655 people, including 155 children, were killed by government forces and its Russian allies in airstrikes.
Rebel fighters and Islamic State killed 98 people in territory held by the government. The Islamic State group also executed 45 people.
Turkish border guards killed 17 people, the report said.