Unidentified warplanes carried out a series of strikes on Friday against rebel-held areas in the north-western Syrian province of Idlib, including a medical facility, a monitoring group reported.
The Syrian Observatory for Human Rights said the strikes targeted the cities Idlib and Ariha, as well as the town of Ariha, where the medical facility was located.
At least 12 people were killed and dozens injured in the bombardment of Idlib city, according to the watchdog, with at least 13 injured in the air raids on Ariha.
The bombardment comes a week after unknown aircraft hit a refugee camp in Idlib, killing at least 30 people.
The cities of Idlib and Ariha are controlled mainly by al-Qaeda's Syria branch al-Nusra Front and allied Islamist rebels.
A member of the non-governmental emergency group White Helmet said the air raids targeted the Douwar Maaret Masreen area of Idlib city, killing 12 people and injuring 47 others.
The dead included three children, according to the aid worker, who asked to be identified only as Majd.
He told dpa that a building housing the offices of the Syrian Red Crescent in Ariha was also hit, without giving casualty numbers.
The Observatory reported that al-Nusra Front and its allies executed at least 19 civilians on Thursday after the rebels overran a key village in central Syria.
The 19 people, including six women, were gunned down inside their homes by the rebels after they took control of al-Zara village in the south of the Hama province on Thursday.
Al-Zara is mostly inhabited by Alawites, a sect to which Syrian President Bashar al-Assad belongs.
The executed people were relatives of fighters loyal to al-Assad's regime, said the Britain-based Observatory, which relies on a network of activists inside Syria.
Islamist rebels have also kidnapped dozens of people from al-Zara whose fate remains unknown.
The village is located near a main road linking Hama and the central city of Homs.
The rebels' attack on al-Zara was reportedly codenamed "Revenge for Aleppo," a reference to the northern city that has been a key battleground between al-Assad's forces and opposition fighters in recent weeks.
Retaliatory attacks by regime forces and rebels were reported in Aleppo on Friday, a day after a temporary truce ended there.
No casualties were reported.
Once Syria's commercial hub, Aleppo is now divided between the government-held west and the rebel-controlled east.
A rights group said on Friday that rebel groups may have committed war crimes during shelling attacks on the Kurdish-controlled area of Sheikh Maqsoud in the Aleppo city.
"The relentless pummelling of Sheikh Maqsoud has devastated the lives of civilians in the area," Amnesty International said.
"A wide array of armed groups from the Fatah Halab coalition has launched what appears to be repeated indiscriminate attacks that may amount to war crimes," Magdalena Mughrabi, the interim deputy director of the Middle East and North Africa Programme at the London-based watchdog, said.
Amnesty said that it had gathered evidence of serious violations and obtained the names of at least 83 civilians, including 30 children, who were killed by attacks in the area between February and April 2016.
The fighting inside Sheikh Maksoud has pitted the Kurdish People's Protection Unit (YPG) against rebel groups backed by foreign countries including Turkey.
Turkey, has been closely monitoring the expansion of the YPG in areas that are close to the Turkish-Syrian border.
Ankara regards the YPG as a terrorist group and fears its expansion in northern Syria.
Around 30,000 civilians are believed to be living in Sheikh Maqsoud, which is a predominately Kurdish part of Aleppo city.
The Sheikh Maqsoud is surrounded from the northern, eastern and western fronts by armed opposition groups who have targeted it from all three sides.