The Syrian Observatory for Human Rights, a Britain-based monitoring group that relies on a network of activists across the war-torn country, said the inter-rebel fighting in Aleppo erupted on Thursday.
It pitted militants of the Al-Qaeda-linked Islamic State of Iraq and the Levant (ISIL) against a battalion linked to the Arab- and Western-backed Free Syrian Army.
"At least 30 fighters from the (FSA's) Ababil Brigade and 14 from ISIL have been killed in combat, and that toll could rise further," the Observatory's Rami Abdel Rahman told AFP.
He said the clashes rocked several districts of Syria's former commercial hub, and that ISIL, which expounds an extreme form of Islam, made gains in three sectors.
The ISIL militants have now consolidated their presence in Aleppo, which has been more or less split into a rebel-held east and pro-regime west since July last year.
Although both claim the ouster of Assad as a common aim, jihadists and the mainstream rebels have come to blows many times in recent months.
Across northern and eastern Syria, ISIL has set up checkpoints on roads to border crossings and opened new fronts to try to crush other groups fighting to oust the embattled president.
The Syrian civil war has drawn in fighters from across the Arab world and beyond since it flared in response to a bloody government crackdown on democracy protests in March 2011.
The Observatory estimates that at least 115,000 people have been killed since then, mostly civilians.
The United States and Russia hope to convene a peace conference next month, but their efforts have been complicated by growing rifts within the Syrian opposition.
The intensifying clashes among rebel groups on the ground come after 13 rebel brigades last month rejected the authority of the National Coalition, the opposition's most prominent political institution.
Mortar fire hits Christian suburb
It said at least four civilians including a child were killed on Saturday when mortar rounds slammed into central Damascus and a suburb of the capital.
At least three people died when several rounds hit Jaramana, a mainly Christian suburb that is considered pro-regime.
And an eight-year-old girl was killed and 11 others were wounded in rebel mortar fire on the Abu Rummaneh district, state news agency SANA reported.
The Observatory said the girl was in a car when she was hit.
Rebels have increasingly targeted central districts of Damascus with mortar fire from rear bases on the outskirts of the city.
On Wednesday, two mortar rounds hit the central bank in Sabaa Bahrat Square, and a week earlier a mortar round hit the Iraqi consulate, killing a woman.
Elsewhere, government forces dropped "barrel bombs" on the strategic town of Sfeireh, near Aleppo, the Observatory said.
The town is largely controlled by jihadists and is near a military site believed to hold some of the regime's arsenal of chemical weapons.
"The regime wants to retake Sfeireh because it wants to take the inspectors to the site," Abdel Rahman told AFP.
"At the moment, they can't, because the road and the area are very dangerous," he said, adding that an exodus of civilians was under way.
Two teams of inspectors are in Damascus verifying and destroying Syria's chemical weapons arsenal under the terms of a UN resolution, a dangerous task requiring them to operate in a war zone.
Underlining the dangers also faced by foreign journalists covering the conflict, a Russian photographer has been abducted by rebels who accuse him of being a spy, the Russian foreign ministry said.
Spokesman Alexander Lukashevich said Konstantin Zhuravlev, 32, was seized in Aleppo and was being held by the Islamist Liwa al-Tawhid group.
France this week identified two more journalists abducted in Syria, bringing to at least four the number of its nationals held in what has become the most dangerous place on the planet for media workers.