United Nations: Syrian forces killed 26 blindfolded men in a stadium and the civilian death toll from protests is now past 2,000, top UN officials told a Security Council meeting, sources said.

Setting out details of President Bashar al-Assad's crackdown, officials said victims as young as 13 were believed to have died in custody and public buildings such as hospitals had become targets of the military assault, sources said.

B Lynn Pascoe, the UN under-secretary-general for political affairs, was among several top UN officials to brief the 15-nation Security Council about events in Syria as international pressure mounted on President Bashar al-Assad.

Pascoe said another 120 people have been killed in the crackdown since the last Security Council discussions on Syria on August 10, taking the toll past 2,000 civilians dead since the protests started in March, according to sources at the closed meeting.

Pascoe added that public buildings such as hospitals had been targeted in the assault by Assad's security forces. He said reforms promised by Assad could not be carried out while the military assault continued, the sources added.

UN High Commissioner for Human Rights Navi Pillay told the council about an alleged incident in a stadium in the protest city of Daraa where 26 blindfolded men were shot dead execution-style.

Pillay, who has said the violence in Syria could amount to crimes against humanity, added that a boy of 13 was among the many missing feared killed in government custody.

The meeting came only hours after US and top European leaders called on Assad to step down.