The UN's special envoy on children in war has arrived 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 during Ramadan, a watchdog reported.

As US- and Russian-backed efforts to convene a Syria peace conference have faltered, regime forces have launched counter-attacks against the fighters in the northwest, in the centre and around the capital.

Leila Zerrougui, the UN secretary general's special representative on children and armed conflict, will spend three days in Syria, the UN said.

She is to meet with government officials, UN representatives and non-governmental organisations, as part of a tour that will also take her to neighbouring Jordan, Iraq, Lebanon and Turkey, the main host countries for the hundreds of thousands of Syrian refugees who have fled the conflict.

According to the Syrian Observatory for Human Rights watchdog, 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.

The deadliest of Sunday's air and artillery bombardments of villages in the northwestern province of Idlib struck Maghara, where 13 people were killed, the Observatory said.

Another six people were killed in the village of Al-Bara, four in Basamis, three in Kfar Nabl in an air strike, and three in Iblin, according to the group, which relies on a network of activists, lawyers and doctors on the ground across Syria.

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