The advance came as Assad's opponents warned his regime's "barbaric and destructive" assault on Qusayr could torpedo US-Russian attempts to organise a conference on ending more than two years of bloodshed.
The Arab League called an emergency meeting for Thursday, ahead of the conference, as the opposition Syrian National Council (SNC) demanded that it meet and "stop the massacre in Qusayr".
Loyalist forces launched today's Qusayr offensive with a heavy early morning bombardment using artillery and warplanes.
He spoke of fierce fighting and said the army quickly seized the southern part of town, the town hall and nearby buildings, and advanced on the outskirts of the western sector of Qusayr.
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"The armed men fled towards the northern sector but we are also advancing on that area to eradicate all armed presence," the soldier said.
He claimed that "100 armed men were killed" in the operation during which troops had to defuse mines and bombs placed by rebels at the gates of Qusayr.
The Syrian Observatory for Human Rights watchdog said Qusayr was struck from the air and pounded with artillery fire before the ground operation began.
Observatory chief Rami Abdel Rahman told AFP troops entered from the south and that Hezbollah militants from Lebanon were "playing a central role".
"If the army manages to take control of Qusayr, the whole province of Homs will fall," he said.
A source close to Hezbollah told AFP in Beirut that four of the group's members were killed overnight shortly before the assault on Qusayr began.