Syrian security forces killed at least 40 people on Saturday as they pursued a crackdown on pro-democracy protesters after activists called for a no-fly zone to protect civilians and soldiers deserting the army, a rights group said.

The Syrian Observatory for Human Rights said all the fatalities, except for one, were in Hama, in the north, and Homs in the centre, and that security forces encircled mosques to prevent protesters from demonstrating after the weekly Muslim prayers.

Syrian forces fired live rounds to disperse protesters, killing at least 40, wounding more than 100 and arresting 500 across the country, the Britain-based Observatory said.

“Twelve civilians were killed in various neighbourhoods of Hama, 20 others in the city of Homs and one civilian was killed in Qusayr, in the region of Homs,” the Observatory said.

Two civilians were also killed and 10 wounded by security forces in Tsil, in the southern province of Daraa, cradle of more than seven months of anti-regime dissent, the group said.

Hama and Homs are at the front line of the anti-regime protests that have rocked Syria since mid-March, since when the UN estimates more than 3,000 people, mostly civilians, have been killed in the bloody repression.

The army has been carrying out operations in Qusayr for several weeks, amid fighting there between troops and suspected army deserters, activists say.

The Observatory’s chief Rami Abdel Rahman said in Nicosia that Homs has seen the “highest number of martyrs to date” since the protest movement unfolded.

“Homs has given 40 percent of the martyrs of the Syrian revolution,” he said.