Syrian troops killed at least 45 civilians in a tank assault to occupy the center of the besieged city of Hama, an activist said on Thursday, in a sharp escalation of a campaign to crush an uprising against President Bashar al-Assad's rule. Thousands of civilians were fleeing the city, a bastion of protest surrounded by a ring of steel of troops with tanks and heavy weapons. Electricity and communications have been cut off and as many as 130 people have been killed in a four-day military assault since Assad sent troops into the city on Sunday, activists say. Reacting to the intensifying assaults on Hama and other Syrian districts, the U.N. Security Council condemned the use of force against civilians -- its first substantive response to nearly five months of unrest in Syria. In Hama, residents said tanks had advanced into the main Orontes Square, the site of some of the biggest protests against Assad, who succeeded his father Hafez al-Assad in 2000. Snipers spread onto rooftops and into a nearby citadel.