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".