Amnesty International says Syrian security forces may have committed crimes against humanity during a deadly operation last month in a town near the Lebanese border.
Citing witness accounts, the London-based rights group accused Syria of rounding up scores of male residents in Talkalakh and torturing most of them, with at least nine people dying in custody.
In a report issued Wednesday, Amnesty said the assault appears to be part of a "widespread, systematic attack against the civilian population," which would constitute crimes against humanity.
The group urged the U.N. Security Council to refer the situation in Syria to the International Criminal Court.
On Tuesday, Syrian government forces opened fire on civilians in the central city of Hama, killing at least 11 people. Activists said the shootings took place after troops moved tanks around the city's outskirts in apparent preparation for an assault.
In response, residents set up dozens of roadblocks and set debris on fire to prevent the advance of tanks currently ringing the town. Hama residents burned tires and trash bins and set up sand barriers and other obstacles to block the expected assault.
Security forces also mounted a second attack Tuesday in northwest Idlib province.
The U.S. and Britain urged Syria to immediately pull its forces from Hama and other cities.
Reuters quoted a French Foreign Ministry spokesman Tuesday as calling on the United Nations Security Council to adopt a firm stance against what he called Syria's "unacceptable, ferocious armed repression."
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.
On Monday, soldiers sealed off the city and raided homes there, one month after government forces withdrew. At least 20 people were arrested as part of Syria's ongoing crackdown on dissent.
Unlike its European partners and the U.S., France says Mr. Assad has lost his legitimacy to rule. But a French campaign for U.N. condemnation of the crackdown has met stiff Russian and Chinese resistance.
But Foreign Minister Alain Juppe - who held talks in Moscow last week - said Tuesday there are signs Russia is beginning to question its Syria policy. Juppe said he attempted to sway his Russian counterpart, Sergei Lavrov, but that Russia is still threatening to veto any U.N. resolution against Syria.
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 during the same period.
Some information for this report was provided by AFP, AP and Reuters.