Human Rights Watch cited reports from witnesses, including doctors, in its report that was released late Thursday.
The accusations stem from a military siege in Homs on Wednesday, when activists said at least 20 people were killed.
The New York-based rights group also says Syrian security forces prevented medical personnel from reaching the wounded in the city that day.
The UN estimates that Syrian President Bashar Assad’s crackdown has killed some 2,200 people since the country’s uprising began in March.