Dozens of people have been injured in clashes with security forces in the Syrian port of Baniyas, where 13 people were killed on Sunday, residents say.
Troops locked down Baniyas and stormed the nearby village of Bayda, they said.
Rights groups say hundreds of people have been arrested, including several students who took part in a rare rally at Damascus University on Monday.
About 200 people have died in weeks of protests against repression by President Bashar al-Assad's government.
The US said the use of force in Syria was "outrageous" and expressed concern about claims the wounded were being denied medical care.
"The United States strongly condemns the continued efforts to suppress peaceful protesters," a statement from the White House said.
The British government warned Britons on Tuesday not to travel to Syria unless absolutely necessary.
The unrest is seen as the biggest challenge to President Assad, who inherited power from his father in 2000.
The flashpoint coastal city of Baniyas remained under lockdown on Tuesday, residents told the BBC.
One witness said the nearby village of Bayda was surrounded by army vehicles.
He said dozens of people had been injured in clashes with security forces, and that soldiers were preventing ambulances from getting into the town.
Other residents said electricity had been cut, the majority of phone lines were down, and essential supplies like bread were in short supply.
Twenty-two people were arrested in Baniyas on Monday, as funerals were held for the four people who died when security forces opened fire on protesters over the weekend.
On Tuesday, Human Rights Watch accused Syrian security forces of preventing medics from reaching wounded protesters when clashes erupted at anti-government demonstrations last week.
"To deprive wounded people of critical and perhaps life-saving medical treatment is both inhumane and illegal," said Sarah Leah Whitson, the organisation's Middle East director.
It said a total of 28 people were killed on Friday when security forces fired on protesters in Deraa, Harasta and Douma, a suburb of Damascus.
Human rights campaigners also say hundreds of arrests have been taking place across Syria.
Fayez Sara, 61, a well-known Syrian writer and journalist, was arrested on Monday - the third opposition figure detained since Sunday.
Others rounded up by security forces include bloggers, activists and young opposition supporters.
Mr Assad blames the violence on armed gangs rather than reform-seekers and has vowed to put down further unrest.
Syria's leading pro-democracy group, the Damascus Declaration, has urged the Arab League to impose sanctions on the regime.
The Damascus Declaration estimates about 200 people have died in the Syrian protests, which first erupted in March in the southern city of Deraa.
The protests then spread despite Mr Assad's offer to consider ending a decades-old emergency law which allows the regime to arrest people without charge.
He has also sought to appease minority Kurds by offering them citizenship, and to defuse anger by sacking local officials.