Rebel mortar shells pounded Syria's northwestern city of Idlib on Monday, killing 14 people, state television said, a day after 27 people died in regime air raids on a nearby opposition-held area.
"Fourteen people were killed and more than 50 others wounded by mortars launched by the terrorists (rebels) on several areas of Idlib city," said the state channel.
The Syrian Observatory for Human Rights confirmed the attack, but gave a higher toll of 15 civilians, and said an unknown number of children were among the dead.
The report comes a day after at least 27 people, including two rebel fighters and four children, were killed in regime air strikes on the rebel-held area of Silqin in Idlib province, according to the Observatory.
Fresh air strikes were launched on Monday in the province, according to activists who have reported an escalation of violence in the area, which is important because it borders Turkey.
Most of Idlib province is in rebel control, though the provincial capital is still firmly in government hands.
Rights groups have frequently denounced the regime's daily air raids on opposition areas across the country, as well as the rebels' use of mortars and rockets, for failing to discriminate between civilians and combatants.
Elsewhere in Syria, the Observatory said fierce clashes raged in Albu Kamal, on the Iraq border, pitting the jihadist Islamic State (IS) against local rebels allied to the Al-Qaeda affiliate Al-Nusra Front, a day after IS announced the establishment of a "caliphate".
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Though IS fighters were initially welcomed in Syria by some opponents seeking President Bashar al-Assad's ouster, the group's abuses and quest for hegemony quickly turned the opposition against it.
IS -- formerly known as the Islamic State of Iraq and the Levant -- has been fighting Syria's rebels since January.
Fighting has raged in Albu Kamal for several days. The town is strategic because it lies on the border with Iraq, where IS has spearheaded a Sunni militant offensive that has seen several areas in northern and western Iraq fall from government control.
On Monday, the Observatory said "reinforcements redeployed to Albu Kamal to strengthen IS' forces".
It also said the regime air force targeted an IS position in Buseira town, also in the oil-rich Deir Ezzor province.
The regime has rarely targeted IS positions, except in recent days since the jihadist group started making gains in Iraq.
The Iraq offensive has bolstered IS's confidence, partly because its jihadists have captured heavy weapons from fleeing Iraqi forces, including US-made armoured vehicles.
Syria's war has killed more than 162,000 people and forced nearly half the population to flee their homes.
It began as a peaceful movement inspired by the Arab Spring demanding Assad's ouster, but morphed into an insurgency after the regime unleashed a brutal crackdown against dissent.