Fierce fighting between Syrian government troops and rebels is taking place in a strategic district of the biggest city, Aleppo.
State media said government forces had taken control of Salah al-Din, but the Free Syrian Army later announced a successful counter-attack.
Images that have emerged from the northern city show buildings reduced to rubble by heavy weapons.
Observers say controlling Aleppo is a crucial goal for both sides.
In another development, Jordanian officials confirmed that former Syrian Prime Minister Riad Hijab crossed into Jordan only on Wednesday – two days after his defection was announced.
Activists said he and his family had been hiding in the south of the country and that reports on Monday that he had already left Syria were aimed at throwing government forces off the trail.
After several weeks of bombardment and skirmishing, it seems the real battle for Aleppo may now be under way. Syrian state television ran urgent news flashes saying government forces were “completely combing” the Salah al-Din quarter on the south-west side of the city.
It said many rebels had been killed, including a number of non-Syrian fighters, and that booby-trap bombs left behind by the armed rebels had been defused. Local commanders of the rebel Free Syrian Army confirmed that a big attack was taking place but denied that the opposition forces had been completely dislodged.
Salah al-Din has been one of the main battlegrounds in the struggle for Syria’s biggest city, and much of the quarter has been very heavily damaged by bombardment.
The BBC’s Jim Muir in neighbouring Lebanon says that Aleppo’s south-western district of Salah al-Din is a densely-packed area of narrow streets and has been one of the city’s main battlefields.
If this is indeed the start of the government’s long-awaited offensive to clear rebels from the city, it is clearly going to take some time, he says.
Salah al-Din is considered a vital supply route for government troops coming from the south.
Earlier on Wednesday, state media said government forces had taken full control of Salah al-Din, killing most of the rebels there.
They also reported heavy rebel losses near the historic citadel and in another district.
But a commander from the rebel Free Syrian Army later said it had launched a counter-attack following the arrival of some 700 reinforcements.
“For an hour and a half the Free Syrian Army has staged a counter-attack and reclaimed three streets out of five seized by regime forces,” Wassel Ayub told AFP news agency by phone.
Another FSA commander, Abdel Jabbar al-Oqaidi, told AFP news agency via Skype: “It is not true the regime army has seized control of the district. It is true that there is a barbaric and savage attack.”
A tweet from journalist Martin Chulov, of the UK’s Guardian newspaper, who is inside the city appeared to support the rebels’ account.
“Spent afternoon in Salahedin [Salah al-Din] #Aleppo. No regime troops inside. Battlelines have not shifted. Lots of shelling and helicopters,” he said.
Videos circulated on Syrian social media on Wednesday show people searching the debris of buildings in a town identified as Tal Rifaat, near Aleppo. One clip shows a plane passing overhead followed by an explosion on the ground.
Earlier, Amnesty International said satellite images had revealed at least 600 probable artillery impact craters in Anadan, also near Aleppo.
It said any attacks against civilians would be documented so that those responsible could be held accountable.
In other developments
- A Russian general whom rebels claimed to have killed in Syria, has turned up alive and well in Moscow. Rebels said Gen Vladimir Kuzheyev was an adviser to the Assad regime
- Photographs published on Facebook appear to confirm reports that Syrian rebels have obtained a heat-seeking, shoulder-fired anti-aircraft missile, the New York Times reports
- Turkish lawyer Osman Karahan, who defended suspected al-Qaeda militants linked to four Istanbul truck bombs in 2003, has died in fighting in Aleppo, Turkish media reported
- Former French President Nicolas Sarkozy appeared to call for outside military intervention in Syria, similar to the operation he authorised in Libya while in office last year
- Iran’s foreign minister said a group of 48 Iranians abducted by rebels in Syria on Saturday included a number of retired Revolutionary Guards and soldiers but denied they played any active military role
- King Abdullah of Jordan suggested Syrian President Bashar al-Assad might try to create an enclave for his Alawite Muslim sect if he loses control of the country