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 the anti-government protests that are threatening his 11-year rule.

World powers have widely condemned the Syrian crackdown.

On Thursday, U.N. Secretary General Ban ki-Moon said he had spoken to Mr. Assad and urged him to “stop killing people.”

U.S. State Department spokeswoman Victoria Nuland called the government crackdown “revolting” and “barbaric.” In a Thursday briefing, she also said the U.S. has been increasing its contacts with Syrians inside and outside of the country who are seeking change.

In Ankara, Turkish officials, faced with caring for more than 8,900 Syrian refugees staying in makeshift tents, pressed an Assad envoy, Hassan Turkmani, to end the military campaign against the protesters.

After a Thursday meeting with the Syrian envoy, Turkish Foreign Minister Ahmed Davutoglu said his country is also extending assistance to about 10,000 people who had massed along the Syrian side of the border.

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 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.