Syrian government forces have increased pressure on two of the last opposition bastions in the country, with heavy air strikes on Idlib province and a move to break a siege on an army base near Damascus.
Syrian and Russian aircraft pressed on with a week-old operation targeting Idlib in the north-west, the last province in the country to escape government control.
Raids on Sunday in the south-east of the province killed at least 21 people, including eight children and 11 members of the same family, said the Syrian Observatory for Human Rights, a Britain-based monitoring group.
"Regime and Russian strikes are continuing today on several parts of Idlib" province, said Observatory head Rami Abdel Rahman.
Russian-backed government forces launched an operation on the edge of Idlib province in the last days of last year and have retaken villages every day since.
Since the collapse of ISIL's hold on territory in both Syria and Iraq late last year, Syrian president Bashar Al Assad's regime has been focused on restoring its grip over the country.
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Idlib province, which borders Turkey, is almost entirely controlled by opposition forces that are dominated by an alliance named Hayat Tahrir Al Sham (HTS), which consists mostly of fighters from Jabhat Fatah Al Sham, Al Qaeda's former affiliate in Syria.
Among the other opposition fighters present in the province are thousands of extremists from Central Asian states and members of the Muslim Uighur ethnic minority of China's Xinjiang province.
An explosion on Sunday in the city of Idlib at a base for the group Ajnad al-Qawqaz, made up of fighters from the Caucasus who operate alongside HTS, killed at least 34 people, including 19 civilians, the Observatory said.
The initial death toll for the attack, the origins of which remain unclear, was 23 but the number went up on Monday when more bodies were found in the wreckage.
Mr Abdel Rahman said the death toll could yet rise because more victims were believed to be buried under the rubble, while many of the wounded were in critical condition.
"Rescue teams are still sifting through the wreckage," he added.
It was not immediately clear whether the blast was caused by air strikes or was the result of the kind of internal clashes that sometimes break out between extremist and other opposition factions.
After shrinking to barely a sixth of the country at the height of the nearly seven-year-old conflict, the areas under government control now cover more than 50 per cent of Syrian territory.
Another pocket where rebels are still holding out, however, is Eastern Ghouta, a semi-rural area east of the capital, Damascus, that is home to some 400,000 people.
Opposition fighters led by the Jaish Al Islam group had in recent days surrounded the army's only military base in the area but the state news agency Sana said on Monday that the siege had been broken.
"Units from the Syrian Arab Army have brought an end to the encirclement of the Armoured Vehicles Base in Harasta," it said, adding that operations were ongoing to fully secure the base.
According to the Observatory, the fighting in Harasta since the base was surrounded in late December has killed 72 government fighters and 87 opposition fighters.
The shelling and bombardment of besieged Ghouta, where the humanitarian conditions have sharply deteriorated in recent months, has also claimed a heavy toll on civilians.
The latest casualties came on Monday when air strikes killed a child and two other civilians in Madeira, a village in Eastern Ghouta, the Observatory said.