Jihadis and other rebel groups made advances against the Syrian army north of Hama on Thursday, a monitor said, part of their biggest offensive for months, underscoring bleak prospects for peace talks that resume later in the day.

A Syrian military source said the government was summoning reinforcements for a counter-offensive. "We have absorbed the attack and we are consolidating defence lines set up in the areas breached," he told Reuters.

Since their Hama assault began late on Tuesday, rebels have captured at least 11 villages and towns from the army, the Syrian Observatory for Human Rights, a Britain-based war monitoring group, said.