A Syrian rights activists says there is intense gunfire in the central city of Homs as security forces continue a crackdown on anti-government protesters.

The activist for the Syrian Observatory for Human Rights said Thursday tanks moved to block off neighborhoods as government forces conducted raids and made arrests.

Syrian rights activists and witnesses have said pro-government forces shot and killed at least 13 people in Homs since Monday. The French news agency reports the number of people killed in Homs has risen to at least 20, including seven who were mourning at a funeral.

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

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

On Wednesday, the United States condemned a Syrian government order prohibiting the U.S. and French ambassadors from traveling outside Damascus without permission. The two diplomats had 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 in order to document a brutal crackdown on anti-government protesters. She noted that the Syrian government refuses to allow international media, aid workers or human rights personnel freedom of access.

The spokeswoman said the U.S. will respond accordingly if the government in Damascus inhibits the ability of diplomats to effectively carry out their responsibilities within Syria.

Earlier Wednesday, Syrian Foreign Minister Walid Moallem announced the restrictions for U.S. envoy Robert Ford and France's Eric Chevallier. He said if they defy the order, Syria will impose a ban on all diplomats leaving the capital.

Ford and Chevallier say they traveled to Hama to show solidarity with residents who are under pressure from the government's crackdown on dissent.