Syrian troops backed by fighters from Lebanon's Hezbollah attacked and entered the strategic rebel stronghold of Qusayr on Sunday, a day after President Bashar al-Assad insisted he would not quit.
The advance came as opponents warned the Assad regime's "barbaric and destructive" assault on Qusayr could render "meaningless" US-Russian attempts to organise a conference on ending two years of bloodshed in the country.
The Arab League called an emergency meeting for Thursday, ahead of the conference, following demands from the opposition Syrian National Council (SNC) for it to meet and "stop the massacre in Qusayr".
The Syrian regime's assault on Qusayr -- located on the key road between Damascus and the Lebanese border -- followed heavy early morning bombardment of the town by its artillery and warplanes.
"The assault on Qusayr has started. There is fierce fighting between rebels and the army around the entrances to the town," said Rami Abdel Rahman of the Syrian Observatory for Human Rights.
"Soldiers and tanks are trying to advance into the town, the rebel forces are attempting to push them back," he told AFP.
Troops were entering from the south, and fighters from the Lebanese militant group Hezbollah, a key ally of the Syrian regime, were "playing a central role," he added.
State television said the army had "tightened the noose on the terrorists, attacking on different fronts and destroying positions of their leaders in the south of the town".
At least 30 people, including 16 rebel fighters, were killed in the fighting, said the Observatory, a Britain-based group that relies on various sources on the ground.
"A rain of shells fell on the city, at the same time as artillery fire and mortar fire from dawn. Homes were destroyed and burnt down," said the Syrian Revolution General Commission, an activist network.
"The Syrian army controls Qusayr's main square in the centre of the city, and the surrounding buildings, including the municipality building," a military source told AFP.
The regime has made recapturing Qusayr and the surrounding district of Homs province a key objective, and fierce fighting has raged in the vicinity for months.
In recent weeks, government troops backed by Hezbollah and members of the National Defence Forces, a pro-regime militia, have taken a string of villages and reportedly surrounded Qusayr on three sides.
Last week, a military source said the army dropped leaflets on Qusayr, warning civilians to flee ahead of an operation, but activists denied this and said there was no safe way out anyway.
The Qusayr area is considered of strategic importance because it lies between the capital and the Mediterranean coast, and is close to the Lebanese border.
Responding to news of the assault on the city, the SNC, a key component of the main opposition National Coalition, denounced the "barbaric and destructive bombing" of Qusayr.
It accused the regime of working with Hezbollah to "invade the town and wipe it and its residents off the map," and called for "an urgent meeting of the Arab League to stop the massacre in Qusayr".
"We say to the countries that are working for a political solution in Syria that allowing this invasion to go ahead in silence... will render any conference and any peace effort meaningless."
The United States and Russia are working to organise a peace conference next month, in a bid to find a political solution to the conflict.
Washington has backed the uprising against Assad, while Moscow is one of his staunchest allies.
But the embattled Syrian leader said in a weekend interview with an Argentine newspaper that he will not resign before the end of his mandate in 2014.
"To resign is to flee," he was quoted as saying by the Clarin newspaper when asked if he would consider stepping down as called for by US Secretary of State John Kerry.
The Syrian military was also advancing on other fronts, taking control of the rebel-held village of Halfaya in Hama province, the Observatory said.
State television reported the army "killed numerous terrorists from Al-Nusra Front in Halfaya" and destroyed weaponry.
In Damascus, a military source said troops were advancing in Barzeh district on the northern outskirts of the city.
The Observatory estimates at least 94,000 people have been killed since the anti-Assad uprising began in March 2011.