The United Nations agency for Palestinian refugees has described its shock and outrage at a bomb attack which killed 18 people near a school it runs in Syria.
The attack came eight days after another explosion injured 40 children at another UN school in the same town.
In a separate report, the UN said that an estimated 240,000 people were trapped in areas under siege in Syria.
Over 100,000 people have been killed in Syria since the fighting began in 2011.
The United Nations Relief and Works Agency for Palestine Refugees in the Near East, known as Unwra, said that the explosion at the school had happened just before midday on Tuesday.
The attack took place at the Zeitoun school in the town of Muzeirib, near the border with Jordan.
The 18 people who died included five children and a nurse, the UN said.
At least 20 people were injured, including two staff members and eight children, the agency said.
Unwra Commissioner-General Filippo Grandi said he felt "revulsion at this wanton disregard for civilian life and international law".
In a separate report released on Wednesday, the UN's Office of the High Commissioner for Human Rights, Navi Pillay, said that an estimated 240,000 Syrians were suffering under local sieges.
The sieges "are imposed by Syrian government forces and pro-government militias, as well as by armed opposition groups", the report said.
"The Security Council is continuing to fail Syrians by not even managing to agree on measures to ensure the provision of basic necessities to people," Ms Pillay said.
Cases of government-enforced sieges have been coupled with shelling and aerial bombardment, the report said.
Another report on Wednesday, by Human Rights Watch, said Syrian government forces were now using a powerful type of cluster munition rocket that had not been seen before in the conflict,
Rebel forces have been fighting President Bashar Al-Assad's troops in Syria since 2011.