Summary Washington has backed the uprising against President Bashar al-Assad.
BEIRUT (AFP) - Syrian troops backed by fighters from Lebanese militant group Hezbollah launched an assault on the rebel-held central town of Qusayr on Sunday, after months of fierce fighting in the area, a watchdog said.
The Syrian opposition condemned "attempts to invade" the town, which it said could render US-Russian attempts to organise a peace conference "meaningless."
"The assault on Qusayr has started. There is fierce fighting between rebels and the army around the entrances to the town," Syrian Observatory for Human Rights director Rami Abdel Rahman said.
"Soldiers and tanks are trying to advance into the town, the rebel forces are attempting to push them back," Abdel Rahman told AFP, adding that troops were entering from the south.
Fighters from Hezbollah, a key ally of the Syrian regime, "are playing a central role in the battle," he said.
State television said that 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."
The ground assault began after a heavy early morning bombardment of the town by aircraft and artillery, which killed at least 20 people, including 11 rebel fighters, the Observatory said.
The Syrian Revolution General Commission, a network of activists on the ground, also reported that Qusayr had come under intense bombardment.
"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," the group said.
The Syrian regime has made recapturing the town of 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 advanced in the region, taking a string of villages and reportedly surrounding Qusayr on three sides.
Last week, a military source said the army dropped leaflets on the town, warning civilians to leave ahead of an imminent military operation.
But activists denied the leaflets were dropped and said there was no safe way out of the city 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 Syrian National Council, a key component of the opposition, 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."
"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 group said in a statement.
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 President Bashar al-Assad, while Moscow is one of his staunchest allies.
