BEIRUT, Lebanon — Syria’s bloody crackdown on protesters — which seemed to signal a new, harrowing chapter in a conflict that has already killed nearly 400 people — provoked growing international concern on Tuesday with calls for the violence to stop and talk of possible sanctions.

Gunfire continued in Dara’a Tuesday after the Syrian Army stormed the restive city with tanks and soldiers a day earlier in an escalation of the counteroffensive against Syria’s five-week-old uprising, according to residents. At least 25 people had been killed there Monday, residents and human rights activists said. .

Witnesses also reported that protesters gathered in apparently impromptu demonstrations on the city’s largely deserted streets despite continued detentions, which numbered in the dozens Monday.

“History will serve as our witness,” one resident, Alaa Hourani, said by phone.

Such was the alarm in the West about developments in Syria, a critical regional player adjacent to Israel and a close ally of Iran, that the United States State Department urged American citizens not to visit the country and said Americans already there should leave immediately.