Syrian security forces have mounted new assaults in the country's restive northwest, sending tanks and troops into two more cities.

Rights activists and witnesses said Friday the Syrian military sent large numbers of soldiers into Maaret al-Numaan, a city of 100,000 on the highway linking the capital Damascus and the country's largest city, Aleppo.

Many of Maaret-al-Numaan's residents fled in recent days as the Syrian troops have sought to crack down on protests in the region against the authoritarian regime of President Bashar al-Assad.

In addition, the security forces moved into the town of Khan Sheikhun.

The attacks came as world leaders are increasingly condemning the crackdown.

On Thursday, United Nations Secretary-General Ban Ki-moon called on the Syrian president to “stop killing people” and negotiate with the protesters “before it's too late.”

Thousands of Syrians have left their homes behind to escape across the border into Turkey. Some said Syrian troops fired random shells at the villages and ransacked their houses.

The latest violence is part of the weeks-long assault by troops loyal to Mr. al-Assad on anti-government protesters seeking to end his 11-year reign. More than 1,300 civilians and 340 government troops have been killed since mid-March.