Pro-government forces in Syria have opened fire on civilians in the central city of Hama, killing at least 11 people.

Activists and residents say the shootings took place on Tuesday after security forces moved tanks around the city outskirts in apparent preparation for an assault. Residents responded by putting up makeshift barricades and setting debris on fire.

Activists say security forces mounted a second assault on Tuesday in the northwest Idlib province.

On Monday, soldiers raided homes in Hama and arrested at least 20 people as part of an ongoing government crackdown on protests.

Hama is one of the centers of opposition to President Bashar al-Assad's 11-year autocratic rule, and was the site of an anti-Assad rally on Friday that drew hundreds of thousands of protesters.

The U.S. State Department on Tuesday urged Syria to immediately pull its forces from Hama and other cities.

Opposition activists say government troops killed three people, including a young boy, on the outskirts of the city Monday.

Rights groups say Syrian security forces have killed at least 1,300 civilians since mid-March while trying to suppress the anti-government uprising. The Syrian government says terrorists and Islamist militants have killed hundreds of security personnel in the same period.