Syrian troops have pushed into a rebel-held town near the Lebanese border, fighting house-to-house and bombing from the air.
The offensive came as President Bashar Assad tried to strengthen his grip on a strategic strip of land running from the capital to the Mediterranean coast.
With the regime scoring gains on the battlefield, the US and Russia could face an even tougher task persuading Assad and his opponents to attend talks on ending Syria's 26-month-old conflict.
Washington and Moscow hope to start talks with an international conference as early as next month, though no date has been set.
Government forces launched the offensive on the town of Qusair just hours after Assad said in a newspaper interview that he will stay in his job until elections - effectively rejecting an opposition demand that any talks on a political transition lead to his exit.
Even though the regime and the main opposition group have not yet committed to attending the conference, UN Secretary General Ban Ki-moon said 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.
The rebels control large rural areas in the north and east of the country, while Assad has successfully defended his hold on the capital, Damascus, the coastal area and parts of Aleppo, Syria's largest city.
Hadi Abdullah, a Qusair activist reached by Skype, said regime troops and Hezbollah fighters began shelling the town late on Saturday, followed by airstrikes early yesterday that forced residents to take cover in basements.
The Britain-based Syrian Observatory for Human Rights, an activist group, said warplanes resumed bombing raids later on Sunday. By the afternoon, regime forces had advanced into the town, engaging in house-to-house battles with rebel fighters, Mr Abdullah said.
Syrian state media said 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 Mr 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.