Around 60 people have been killed in Syria's Damascus and Homs, as an international watchdog said it would probe alleged chlorine attacks in the country.
Meanwhile, the parliament speaker said four new candidates had registered for next month's presidential election, expected to return Bashar al-Assad to office despite the civil war, which has left vast swathes of the country out of his control.
A barrage of mortar shells fired by rebels hit a central neighbourhood in the capital early Tuesday, killing at least 14 people, state media reported.
The attack hit a school of Islamic jurisprudence where some students are as young as 14, though it was unclear if children were among the dead.
The Syrian Observatory for Human Rights, a Britain-based monitoring group, put the toll at 17, adding that the figure could rise because several of the injured were in critical condition.
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Hours later, a car bomb ripped through a crowded area of the country's third city Homs, followed shortly afterwards by a rocket attack on the same neighbourhood, the provincial governor Talal Barazitold AFP.
He said 45 people were killed in the double attack on the Zahra neighbourhood.
The attack was one of the deadliest to hit the central city, where rebels control just a few remaining districts, most of them under a tight government siege.
Meanwhile the jihadist Islamic State of Iraq and the Levant announced it had executed seven prisoners in its bastion in northeastern Syria, two of them by crucifixion.
ISIL, which has been disavowed even by Al-Qaeda, said it held the seven responsible for a grenade attack on one of its fighters earlier this month in the Euphrates Valley city of Raqa, which it rules with an iron fist.
The Britain-based Syrian Observatory for Human Rights posted a photograph of the two prisoners being crucified at the roundabout with passer-by walking past apparently unfazed.
In the Hague, the Organisation for the Prohibition of Chemical Weapons (OPCW) said it would examine allegations that chlorine had been used in attacks in Syria.
The regime and rebels have blamed each other for using chlorine in at least one attack, in the rebel-held town of Kafr Zita in Hama province, with the opposition alleging the government has carried out several more.
Britain's Daily Telegraph said a team of unnamed experts had found "sizeable and unambiguous traces of chlorine and ammonia" in soil samples taken from the sites of three regime helicopter attacks.
It said the findings proved the government was still using chemical agents against civilians.
The OPCW is already in Syria overseeing a deal under which Damascus is to turn over its chemical weapons arsenal by June 30.
On Sunday, the joint UN-OPCW mission in Damascus said 92.5 per cent of the country's chemical weapons material had been removed or destroyed.
Syria agreed to dismantle its chemical weapons program last year, after Washington threatened military action in response to a sarin gas attack outside Damascus that killed up to 1,400 people.
The regime denied carrying out the attack.
