Rights campaigners said the dead were among thousands of protesters who demonstrated after Friday prayers in cities across the country, from Banias on the Mediterranean coast to Qamishly in the Kurdish east, demanding an end to President Bashar al-Assad's rule.
The European Union agreed to impose sanctions in response to Assad's violent crackdown on protesters, which rights campaigners say has killed more than 580 people.
"The United States believes that Syria's deplorable actions toward its people warrant a strong international response," White House press secretary Jay Carney said in a statement.
"Absent significant change in the Syrian government's current approach, including an end to the government's killing of protesters ... the United States and its international partners will take additional steps to make clear our strong opposition to the Syrian government's treatment of its people."
The United States imposed sanctions of its own last week against some figures in the Syrian government.
Friday's bloodiest confrontation was in the city of Homs where 15 protesters were killed, activist Ammar Qurabi said.
State television said an army officer and four police were killed in Homs by a "criminal gang," though another activist, Wissam Tarif, said witnesses told him nine soldiers defected in Homs to the protesters and may have clashed with other troops.
Four protesters were killed in Deir al-Zor, said a local tribal leader from the region which produces most of Syria's 380,000 barrels per day of oil. They were the first deaths reported there in seven weeks of nationwide unrest.
International criticism has mounted against Assad, who has gone on the offensive to maintain his family's four-decade grip on power in the country of 20 million and crush demonstrators demanding freedom. Read More