Syrian government troops have pushed into the rebel-held town of Qusair near the Lebanese border.
The army launched the offensive on the town just hours after Syrian president Bashar al-Assad said in a newspaper interview that he will stay in his job until elections.
Hadi Abdullah, a Qusair activist reached by Skype, said regime troops and Hezbollah fighters began shelling the town late Saturday, followed by airstrikes early yesterday that sent residents taking cover in basements.
The Britain-based Syrian Observatory for Human Rights, an activist group, said warplanes resumed bombing raids later Sunday.
By the afternoon, regime forces had advanced into the town, engaging in house-to-house battles with rebel fighters, Abdullah said.
Syrian state media said Mr Assad's troops took control of the main square, the area around the municipal building, a sports stadium and a local church. Syrian state TV said troops arrested rebel fighters who tried to flee Qusair dressed as civilians.
A government official said the regime left an escape road open to civilians, a claim denied by Abdullah, who said thousands of non-combatants were trapped in Qusair.
"We tried to get civilians out four times. They are not allowing us," he said of regime forces.
The Observatory said 52 people were killed in Qusair, including 48 fighters, three women and a male civilian.
The main political opposition group, the Syrian National Coalition, said some 40,000 civilians are currently in Qusair and expressed concern for their safety.
Even though the regime and the main opposition group have not yet committed to attending international talks aimed at ending the conflict next month, UN Secretary General Ban Ki-moon said Sunday that he is hopeful it can take place "very soon," possibly in early June.
In addition to the US and Russia, he said he has spoken with Britain, France, China and other key parties.
Previous diplomatic initiatives have failed, in part because of divisions within the international community and because the regime and the armed opposition believed they could achieve more on the battlefield than in talks.
Russia and the US have backed opposite sides in Syria.
Before this weekend’s military push, Qusair had been ringed by regime troops and fighters from the Lebanese militia Hezbollah, an Assad ally, for several weeks.
Qusair lies along a land corridor between Damascus and the Mediterranean coast, the heartland of Assad's Alawite sect, an offshoot of Shia Islam.
Many rebel fighters are Sunni Muslims and Qusair, overwhelmingly Sunni, had served as a conduit for shipments of weapons and supplies smuggled from Lebanon to the rebels.