Syrian rights activists say the country's security forces are expanding their assault in the northwest region that for days has been under a military crackdown.

The London-based Syrian Observatory for Human Rights said Thursday that the Syrian military used dozens of tanks, armored cars, personnel carriers and army trucks to begin an assault on Khan Sheikhun in Idlib province.

The Khan Sheikhun military operation is the latest in the weeks-long campaign by President Bashar al-Assad to suppress anti-government protests that threaten his 11-year rule that followed a 29-year reign by his late father, Hafez al-Assad.

Western leaders have widely condemned the Syrian crackdown.

In Ankara, Turkish officials, faced with caring for more than 8,500 Syrian refugees who have fled their homeland to stay in makeshift tents just inside Turkey, pressed an Assad envoy, Hassan Turkmani, to end the military campaign against the protesters.

Meanwhile, the U.S. State Department said it “condemns in the strongest possible terms” the Syrian use of force against peaceful demonstrators. It said the “outrageous use of violence to quell protests must come to an end now.”

On Wednesday, hundreds of Syrians fled the northern city of Maarat al-Numan ahead of a feared military assault. Witnesses and activists in Maarat al-Numan say Syrian troops intermittently shelled areas near the city and raided nearby villages, making arrests.

Witness accounts say army units have also surrounded the eastern town of of al-Boukamal near the Iraqi border, where clashes have broken out between protesters and pro-Assad loyalists. Forces also entered nearby Deir al-Zour, one of Syria's largest cities, where protests continued.

Rights activists say 1,300 civilians and 340 security force members have been killed since the protests against Mr. Assad's government began in mid-March. About 10,000 people have been detained.