More foreign fighters claiming allegiance to al-Qaida have reportedly joined rebels in war-torn Aleppo, deepening the regional implications of the conflict in Syria.
According to Zeina Khodr, a journalist with news channel Al Jazeera, "Arab fighters from Saudi Arabia and Egypt who didn't want to be filmed" were operating in the city. Some reportedly claimed allegiance to al-Qaida.
Earlier this month, NBC News chief foreign correspondent Richard Engel witnessed al-Qaida units crossing the border from Turkey into Syria, raising fears that militant Islamists could use the civil war to establish a power base in the heart of the Middle East.
Russia warns Obama against' violation' of law over Syria
The 16-month Syria crisis has widened in recent weeks, with deadly sectarian clashes spreading over the border into Lebanon's coastal city of Tripoli.
At least 13 people have died and more than a hundred have been wounded in fighting this week between Lebanese Sunni Muslims and Alawites, including one in the early hours of Thursday.
NBC's Richard Engel answers your questions about Syria
Government troops and opposition forces have been fighting in Aleppo for a month after rebels took over several neighborhoods.
Human rights group Amnesty International on Wednesday artillery and mortar fire and airstrikes by President Bashar Assad's forces in the northern city are killing mostly civilians, including children.
Clashes over Syrian conflict in Lebanon leave 10 dead
Meanwhile, Syrian forces backed by tanks stormed Daraya, on the outskirts of Damascus, on Thursday after 24 hours of artillery and helicopter bombardment to drive out rebels, opposition sources told Reuters.
The bombardment killed at least 15 people and wounded 150 in Daraya, situated on the southwest edge of Damascus, the sources said.
Main players in Syria's civil war
Troops were conducting house to house raids in the conservative Sunni Muslim town and making their way to the town's centre, meeting light resistance from rebels who appear to have largely withdrawn from the area, activists in Damascus said.
"They are using mortar bombs to clear each sector then they enter it, while moving towards the centre," said Abu Zeid, one of the activists. He was speaking by phone from an area adjacent to Daraya.
Other activists said the army was also shelling parts of the suburb from Qasioun, which overlooks Damascus, and from Republican Guard barracks situated near a hilltop presidential palace.
Tim Marshall, a journalist with UK news channel Sky News, said he counted 32 explosions in the space of just one hour in the southern suburbs of Damascus early on Thursday.
Londoner against Londoner: UK fighters held journalist captive in Syria
Britain on Wednesday echoed the warnings of President Barack Obama that the use or threat of chemical weapons by the Syrian regime would force them "to revisit their approach".
A spokesman for U..K Prime Minister David Cameron said the "appalling situation that continues in Syria" had been the main focus of a telephone conversation between Obama and Cameron, according to a BBC report.
Obama on Monday threatened "enormous consequences" if his Syrian counterpart used chemical or biological arms or even moved them in a menacing way.
The president used some of his strongest language yet to warn Assad not to use chemical or biological weapons – after Syria acknowledged for the first time that it had such weapons and could use them if foreign countries attacked it.
Reuters contributed to this report.
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