Syrian security forces and gunmen loyal to the regime shot dead 11 people Tuesday as residents erected roadblocks to prevent the advance of tanks ringing the city of Hama, which has become a flashpoint of the uprising against autocratic President Bashar Assad, activists said.

Hama residents burned tires, set up sand barriers and other obstacles to block the military, said Rami Abdul-Rahman, the London-based director of the Syrian Observatory for Human Rights.

"There is an open civil defiance in Hama," Abdul-Rahman told The Associated Press. "There is a kind of determination not to submit to any tanks or military vehicles."

He said at least 11 people were confirmed dead, citing accounts from doctors and witnesses.

On Monday, Syrian forces sealed off Hama and blocked the roads leading in, an apparent attempt to retake the city one month after security forces withdrew from it. About 300,000 people protested against the regime in Hama last week, a sign the city was spiraling out of government control.

Hama, which has a history of militancy against the Assad regime, was targeted by Assad's father and predecessor in a major government crackdown nearly three decades ago.

In 1982, the late Hafez Assad ordered his troops to crush a rebellion by Sunni fundamentalists, killing between 10,000 and 25,000 people, rights groups say.

The 14-week uprising against Assad has proved remarkably resilient despite a deadly government crackdown that has brought international condemnation and sanctions. Assad is facing the most serious challenge to his family's four decades of rule in Syria.

Activists say security forces have killed more than 1,400 people -- most of them unarmed protesters -- since mid-March. The regime disputes the toll, blaming "armed thugs" and foreign conspirators for the unrest.

Assad has promised a series of reforms that would have been unthinkable before the uprising, which was inspired by the revolutions sweeping the Arab world. He lifted the country's reviled emergency law, which gave the state a free hand to arrest people without charge, and said a national dialogue would start soon.

But the protesters, enraged by a growing death toll, are increasingly calling for nothing less than the downfall of the regime.

Syria has banned nearly all foreign media and restricted media coverage, making it nearly impossible to independently verify events on the ground. But witness accounts, including interviews with refugees who have fled to neighboring countries, indicate that the regime is cracking down hard on the protest movement.

The U.S. and Britain said Tuesday that amassing forces outside Hama casts doubt on Syrian regime claims that it is seeking dialogue with protesters.

"We'd like to see their forces pulled back from cities, from the border. We'd like to see the violence stop," said U.S. State Department spokeswoman Victoria Nuland. "We'd like to see peaceful demonstrators allowed, and we'd like to see a real dialogue begin. We want to see political prisoners released. We want to see repression and torture ending in Syrian jails."

British Foreign Secretary William Hague said violent repression further undermines the legitimacy of the Assad regime.

"The UK has made clear that President Assad must reform or step aside," he said. "If the regime continues to choose the path of brutal repression, pressure from the international community will only increase."

Also Tuesday, a Syrian activist said buses carrying security forces had been spotted heading to restive, mountainous areas near the Turkish border. Omar Idilbi, a spokesman for the Local Coordination Committees, which track the protests in Syria, said witnesses told him the vehicles were rushing to the area where the military has been trying to prevent the opposition from establishing a base.

About 10,000 Syrians have fled to Turkey amid the crackdown.

The exodus has been a source of embarrassment to Syria, which has tried to tightly control coverage of the revolt. It also has strained ties with Turkey.

On Tuesday, the head of the Syrian Red Crescent, Abdurrahman Attar, urged all Syrian citizens in Turkey to return home, Turkey's state-run Anatolia news agency said. Attar visited the Turkish city of Hatay along the countries' border but did not visit the refugee camps, the report said.

"Our wish is to see our citizens to voluntarily return Syria. I have been promised that their lives will be protected. We expect Syrians to return home as soon as possible," he said.