The death toll from airstrikes on a rebel-held area near the Syrian capital Damascus on Thursday has risen to 23 civilians, a monitoring group reported.
The Syrian Observatory for Human Rights said that the deaths resulted from 14 strikes mounted by unknown warplanes in the village of Deir al-Assafeer in the Eastern Ghotta region on the outskirts of Damascus.
Four children were among the dead, according to the Britain-based watchdog.
Initial reports had put the death toll at 10.
Forces of Syrian President Bashar al-Assad have been trying to retake the village for weeks.
The targeted area is covered by a partial ceasefire brokered by world powers including the United States and Russia.
Activists in the area said the air raids hit a school and a field hospital.
There was no official comment in Damascus.
The Islamist rebel group Jaish al-Islam (The Army of Conquest) that controls the area, called the attack a "massacre" and vowed retaliation.
The main opposition group, the Syrian National Coalition, denounced the attack.
"The international community has to take real and urgent action against this massacre that violates the truce," the Istanbul-based bloc added in a statement.
The ceasefire, which went into effect in February, excludes the Islamic State terrorist militia and al-Qaeda's branch in Syria, the al-Nusra Front.
In recent months, a US-led military alliance has targeted extremists in Syria and neighbouring Iraq.
The Observatory also reported on Thursday that an unidentified drone killed a senior Islamic State commander in the group's de facto capital of al Raqqa in north-eastern Syria.
Abu al-Hija al-Tunisi was killed on the edge of al Raqqa, shortly after he was sent by the Islamic State chief Abu Bakr al-Baghdadi to command fighting in the northern province of Aleppo, the watchdog added without giving details.
Last week, the US said it had killed Abdul al-Rahman Mustafa al-Qaduli, a top Islamic State leader who oversaw the group's financial operations.
Washington had placed a 7-million-dollar reward for al-Qaduli, and described him as a senior Islamic State leader who returned to the terrorist group after being released from an Iraqi prison in 2012.