The Syrian military launched an assault Wednesday on a strategic and highly symbolic rebel stronghold in the northern city of Aleppo, signaling a major government effort to reassert control of the nation's commercial hub.
There were conflicting accounts about which side, if either, had the upper hand.
Syrian state television reported that the government had assumed "full control" over Aleppo's Salahuddin district, the target of weeks of military bombardment.
It reported the deaths of many "terrorists," the government's label for the armed rebels fighting to oust President Bashar Assad.
However, rebel commanders and opposition spokesmen said the battle was ongoing and denied that insurgents had pulled back from Salahuddin.
"Our boys are still there," said Mahmoud Sheik Elzoor, a rebel commander. "What the government is saying is not true."
In a video posted online, a commander from the rebel Tawheed Brigade vowed, "We won't leave here till we die."
The frontline neighborhood, parts of which have been reduced to rubble by the bombardment, is a strategic entry point to Syria's most populous city.
For months, while much of Aleppo was quiet, Salahuddin was a center of antigovernment protests. Many residents of the working-class district trace their ancestry to nearby rural areas, where opposition to Assad's government is intense.
Most inhabitants have fled Salahuddin since the fighting began almost three weeks ago. Some have escaped to nearby Turkey, joining an exodus that has accelerated in recent days.
The opposition repeatedly has vowed not to abandon Salahuddin, eschewing the "strategic withdrawal" tactic often used in guerrilla warfare. Earlier this year, rebels executed such a withdrawal from the Baba Amr district of Homs, which endured weeks of shelling.
It was unclear whether the government assault Wednesday represented the army's long-awaited, all-out onslaught or was a limited foray focusing on the Salahuddin area.
The government has professional troops, tanks and aircraft at its disposal against ill-trained rebel formations mostly armed with rifles and rocket-propelled grenades. Rebels lack heavy weapons and are running low on ammunition.
In Washington, President Obama's counter-terrorism adviser said publicly for the first time Wednesday that the administration was studying the possibility of creating a no-fly zone in Syria to prevent airborne attacks on civilians, as well as other steps toward some form of U.S. military involvement.
Creating a no-fly zone over Syria would face considerable hurdles. Russia, long an ally of Assad, would probably object to any effort to win U.N. approval for such a plan. Syria's antiaircraft capabilities are said to remain robust and would have to be neutralized in what could be a long bombing campaign.