The UN's special envoy on children in war was in Syria for talks as concern mounted over the rising child death toll in the bloody two-year conflict.
Six children were among 29 people killed in a devastating army bombardment of five villages in the northwest as residents prepared to break the daytime fast observed by Muslims during the holy month of Ramadan, a watchdog reported on Monday.
As US- and Russian-backed efforts to convene a Syria peace conference falter, regime forces have launched counter-attacks against the rebels in different parts of the country.
Leila Zerrougui, the UN secretary general's special representative on children and armed conflict, will spend three days in Syria, where she will meet government officials, UN representatives and non-governmental organisations, the United Nations said.
She will also visit neighbouring Jordan, Iraq, Lebanon and Turkey, which are hosting hundreds of thousands of Syrian refugees.
According to the Syrian Observatory for Human Rights, more than 100,000 people have been killed since the uprising began in March 2011, more than 5,000 of them children under the age of 16.
Of the 29 killed in Sunday's air and artillery bombardments of villages in the northwestern province of Idlib, at least eight were women and six were children, the Observatory said.
The Britain-based group relies on a network of activists, lawyers and doctors on the ground across Syria for its information.
Video footage posted online by activists showed harrowing scenes of death and destruction in the village of Maghara, where the Observatory says 13 people were killed.
Survivors were screaming as the camera panned over the rubble.
"God is great. Where are our Muslim brothers? Where are our Arab brothers?" the activist says as he films residents trying to dig out people trapped beneath the wreckage of their homes.
"This is the iftar of the Muslims in Jabal Zawiya," he said, referring to the hill district where the village lies.
A second video showed smoke billowing over the village and residents lifting a dust-covered older man, his stomach torn open, onto a flat-bed truck.
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Another man lay dead on the ground, his body and clothes covered in grey dust flecked with blood, his mouth open, his arm curled upwards and his hand lying on his chest.
The attacks came as the army pressed an offensive in the Damascus district of Qaboon, where the regime is trying to uproot several rebel rear bases.
The Observatory said at least 18 people were killed in the fighting.
On Sunday, the group warned that hundreds of families were trapped in the district because regime snipers were posted on the outskirts. There was no immediate update on their plight.
In a statement, the Syrian opposition National Coalition accused regime troops of using residents of the districts as human shields.
"Humanitarian corridors must be established immediately to evacuate women, children, and the wounded in Qaboon," the group said.
On Monday, the pro-government Syrian newspaper Al-Watan reported fierce fighting between the army and rebels over the weekend.
"The army has stressed that operations in Qaboon pave the way for ridding the neighbourhood of militants, who have lost most of their sites because of the army's actions."
President Bashar al-Assad's regime has made eradicating rebel rear bases in the Damascus region a priority as it seeks to prevent the insurgents from attacking the capital.
Nationwide, at least 129 people were killed in Sunday's violence, the Observatory said.
At the UN meanwhile, the United States led Western calls for tougher UN action on Iranian arms supplies to Syria and its Lebanese ally Hezbollah.
The seizure of Iranian arms off the Yemen coast in January "was more than just a sanctions violation, it was an aggressive act to undermine Yemen's transition," acting US ambassador Rosemary DiCarlo told a Security Council meeting.
The vessel was intercepted by the Yemeni coastguard in the Arabian Sea on January 23.