Rockets killed at least 18 people on Wednesday in regime-held areas of Syria's main northern city of Aleppo, a focal point of the 33-month conflict pitting loyalists against rebels.
On the political front, a Syrian minister said President al-Assad would remain president and lead any transition agreed upon at Geneva peace talks planned for next month, despite the opposition's demands he be excluded from the process.
The multiple rocket attack on the Furqan and Meridian districts of Aleppo, the country's pre-war commercial capital, also wounded at least 30 people, the Syrian Observatory for Human Rights said.
State television said at least 17 people were killed in what it described as an attack by "terrorists," the regime term for the rebels fighting to oust Assad.
Aleppo has been one of the main battlegrounds of the Syrian conflict since rebel fighters seized large swathes of the city in an offensive launched in July last year.
But despite persistent skirmishes between the loyalist and rebel forces, the front lines have changed little in more than a year, reflecting a stalemate across much of war-torn Syria.
The international community has become increasingly alarmed about the potential spillover into neighbouring countries of the war that has killed an estimated 126,000 people since it erupted in March 2011.
And Wednesday's rocket attack came as Hezbollah said one of its top leaders was killed near Beirut at a time of soaring sectarian tensions in Lebanon over the conflict in its larger neighbour.
"The Islamic resistance announces the death of one of its leaders, the martyr Hassan Hawlo al-Lakiss, who was assassinated near his house in the Hadath region" east of Beirut, Hezbollah said.
It blamed Israel for the assassination of Lakiss, who was part of the powerful Shiite movement's secretive top leadership.
"Direct accusation is aimed of course against the Israeli enemy which had tried to eliminate our martyred brother again and again and in several places but had failed, until yesterday (Tuesday) evening.
"This enemy must bear full responsibility for and all the consequences of this heinous crime," Hezbollah said on its Al-Manar television channel, without elaborating.
Israeli flatly rejected the allegations.
Foreign ministry spokesman Yigal Palmor described the charges as "yet another Pavlovian response from Hezbollah, which makes automatic accusations before even thinking about what's actually happened."
"Israel has nothing to do with this," he told AFP.
Hezbollah openly admitted seven months ago that it is fighting alongside Assad's forces, fuelling sectarian tensions in Lebanon, a country bitterly divided over the war.
The conflict flared nearly three years ago with peaceful pro-democracy protests inspired by the Arab Spring but escalated into a full-scale civil war after Assad's regime launched a brutal crackdown.
Today hundreds of armed groups, including powerful jihadist brigades affiliated with Al-Qaeda, are battling both the regime and each other, complicating any efforts to reach a political settlement.
On Wednesday, Information Minister Omran al-Zohbi insisted Assad would remain president and lead any transition agreed at the Geneva talks scheduled for January 22.
"If anyone thinks we are going to Geneva 2 to hand the keys to Damascus over (to the opposition), they might as well not go," he said in remarks carried by the official SANA news agency.
The conference is envisioned as a follow-up to the Geneva 1 meeting of June 2012, at which the two sides agreed on the formation of a transitional government without specifying what role, if any, Assad would have in it.
The plan was never implemented.
Meanwhile, a Syrian government daily accused rebels of kidnapping a dozen nuns from a convent in the Christian town of Maalula, north of Damascus, to use them as "human shields".
The rebels took the Syrian and Lebanese nuns to a nearby area under their control after capturing the historic town on Monday, in a case that prompted Pope Francis to call Wednesday for prayers for them and others kidnapped in the conflict.
In a joint statement with the opposition National Coalition, the mainstream rebel Free Syrian Army denied any involvement by its fighters in the nuns' abduction.
"We are doing all we can to get them taken to somewhere safe from fire by regime militia," the statement said.