Syrian forces using tanks and other weaponry intensified their crackdown on anti-government protesters in the central city of Homs on Thursday.

Residents and activists say Syrian tanks moved to block off neighborhoods as government forces conducted raids and made arrests. Shots were heard.

Syrian rights activists and witnesses say pro-government forces have killed at least 20 people in Homs since Monday, including seven mourners at a funeral.

The city is a center of recent protests against President Bashar al-Assad.

It is difficult to verify accounts of the violence in Syria because its government has barred foreign media from reporting and traveling freely in the country.

Meanwhile, France and the United States have condemned a Syrian government warning that the U.S. and French ambassadors should not travel outside Damascus without permission. The two diplomats angered the Syrian government by visiting the flashpoint city of Hama this month.

A U.S. State Department spokeswoman said diplomats must be allowed to travel freely throughout Syria so they can document a brutal crackdown on anti-government protesters. She noted that the Syrian government refuses to allow freedom of access to international media, aid workers, and human rights personnel.

She said the United States will respond accordingly if the Syrian government inhibits the ability of diplomats to effectively carry out their responsibilities.

Syrian Foreign Minister Walid Moallem announced the restrictions for U.S. envoy Robert Ford and France's Eric Chevallier earlier Wednesday.