Air strikes by regime warplanes killed at least 12 children in two incidents on Sunday, one targeting a Kurdish village in northeast Syria and the other a district of Damascus, a watchdog said.
Elsewhere, activists accused the regime of destroying the minaret of the historic Omari mosque in southern Daraa, the so-called cradle of the uprising against President Bashar al-Assad.
"Sixteen people were martyred after a warplane targeted the village of Haddad, which is majority Kurdish... including at least three children and two women," the Syrian Observatory for Human Rights said.
Video footage uploaded on YouTube showed the aftermath of the attack in the province of Hasakeh, with several women carrying children from a damaged home, outside which two bodies can be seen, a pool of blood next to the head of one.
In northern Syria, the Kurdish population has largely observed a careful compromise with regime and rebel forces, fighting alongside neither, in return for security and semi-autonomy over majority Kurdish areas.
But there have been reports in recent weeks of Kurdish forces fighting alongside rebels in certain areas, and Observatory director Rami Abdel Rahman told AFP that rebels had used an area some distance from the site of the raid as a gathering point.
In the second incident, at least nine more children were killed in an air strike on the Qabun neighbourhood of northeastern Damascus, said the Britain-based Observatory.
Video of what activists said was the aftermath of the strike showed part of a house reduced to rubble and the surrounding air thick with dust raised by residents as they picked through grey breezeblocks and steel joints looking for survivors.
A second video also uploaded by activists on YouTube showed the bodies of at least seven children in blue body bags, some small enough to be sharing the bag with one or two others.
In the south, activists accused the regime of destroying the minaret of the historic Omari mosque in Daraa, the so-called cradle of the uprising.
In amateur video footage, the mosque can be seen at the end of a street, its towering minaret toppling over after apparent shelling and crumbling into rubble and dust.
But state news agency SANA quoted a local official in Daraa pinning blame for the minaret's destruction on the Islamist rebel group Al-Nusra Front.
"Al-Nusra terrorists in Daraa targeted the minaret," the unnamed official said, adding that they had obtained a fatwa, or religious edict, authorising attacks on places of worship "if necessary."
"All the signs prove that the terrorists blew up the minaret," he said.
Earlier this week, Al-Nusra's chief pledged allegiance to Al-Qaeda, a day after Al-Qaeda in Iraq announced its affiliation with the Syrian jihadist group, prompting concern among some rebel fighters and opposition members.
On Sunday, the key Syrian National Coalition grouping warned that Al-Nusra Front's pledge of fealty to Al-Qaeda would serve the Assad regime.
"The Syrian Coalition is deeply concerned about recent statements regarding the affiliations and ideologies of particular factions of the rebel forces," the group said.
"Such initiatives only serve the goals of the Assad regime and harm the progress of the revolution," it added, calling on Al-Nusra "to stay within the ranks of nationalistic Syrians."
Violence raged on in the city of Aleppo, where Syrian media said 18 people were injured, including three state television journalists, when a car bomb exploded in the western Furqan district.
Throughout Syria on Sunday, at least nine rebels and 44 civilians were killed, including 23 children, the Observatory said in a preliminary toll.