TALBISEH, Syria -- This city is almost completely empty after a week of heavy shelling by the Syrian government. But it is empty of government forces as well.

The shelling, which killed 20 civilians and five rebel fighters, failed to dislodge rebels who had driven the Syrian military out of nearly a dozen bases and checkpoints in the city over a period of two days earlier this month.

Syrian president Bashar Assad has promised a new offensive to drive rebels from their strongholds in the center of Homs, the country’s third largest city, as the civil war in Syria grinds on. Opposition activist groups reported that Syrian troops bombarded Homs on Sunday with mortar rounds and heavy machine guns, leaving at least 11 people dead.

The site of the worst violence in the country since a peaceful uprising against Assad became an armed rebellion last year, Homs has been preparing for more fighting as Syrian troops massed around rebel-held neighborhoods there over the weekend. But the troop buildup belied the fact that more of the countryside around Homs – which includes Talbiseh – had fallen out of the government’s control.

The rebel offensive and the Syrian military’s heavy shelling prompted the United Nations on Saturday to call off its efforts to monitor a cease-fire that was supposed to take effect in April but never did. The head of the U.N. mission, Maj. Gen. Robert Mood, the chief of the U.N. observer mission in Syria, cited clashes over the past 10 days that were “posing significant risks” to the force of 300 unarmed observers.

Syrian troops were also driven out of Rastan, a city of similar size to Talbiseh about 10 miles to the north, earlier this month. The victory came at a heavy cost as some parts of the city were entirely destroyed. Both Rastan and Talbiseh lie on the north-south highway that is the country’s main artery and connects the capital of Damascus, south of Homs, to Aleppo, the country’s largest city and economic hub, in the north.

“We have a plan to control this area,” said Abdel Rizaq Tlass, the leader of the Farouq Brigade, one of the largest groups of rebels in Homs province that operates under the umbrella of the Free Syrian Army, the name taken by the majority of armed rebels in Syria.

The fighting around Homs suggests Syrian troops in the area are largely demoralized as highly motivated and increasingly well armed and organized rebel forces go on the offensive. The fighting in Talbiseh involved hundreds of rebels.

“We didn’t imagine they had these numbers and so much equipment,” said a man who identified himself as a Syrian army captain who surrendered to the rebels in Talbiseh last week and was allowed to join them. He asked not to be named for his protection.

“Many of the soldiers ran away when the attacks began,” said Mahmoud Najjar, a spokesman for the Farouq Brigade in Talbiseh.

Farouq’s group of fighters in Qusayr, southwest of Homs, launched attacks on checkpoints all over that city on Friday and Saturday, reportedly driving the military out of the southern part of the city. Qusayr, the largest city between the Lebanese border and Homs, had been split between the rebels and the government’s forces for months.

The takeover of Qusayr was impossible to verify independently.

In both Rastan and Talbiseh, destroyed Syrian tanks and armored personnel carriers remained on the streets – including one near Talbiseh that remained in the median of the highway.