Syrian rights activists and residents say government tanks and troops have entered the central city of Hama, making arrests in a renewed crackdown on the hotbed of opposition to President Bashar al-Assad.

The activists and residents say gunfire was heard in Hama on Wednesday, as tanks and military vehicles drove into the city. They say hundreds of Syrian security forces also emerged from buses parked on the city's outskirts and entered on foot, searching for activists involved in a five-month pro-democracy uprising.

Syrian security forces withdrew from Hama earlier this month following a weeklong siege of the city, which has seen some of the country's biggest protests demanding an end to Mr. Assad's 11-year autocratic rule.

The United States expanded sanctions against Syria on Tuesday, in response to Mr. Assad's violent crackdown on the uprising.

The new measures include asset freezes and bans on business interactions with Syrian Foreign Minister Walid Muallem, Assad adviser Bouthaina Shaaban, and Syrian ambassador to Lebanon, Ali Abdul Karim Ali.

A State Department spokeswoman said Tuesday the three were targeted because of their role in propagating what she called the “reign of terror” that President Assad has unleashed on his people.

The Syrian government has blamed much of the country's violence on what it calls armed gangs and terrorists.