BEIRUT — Syrian President Bashar Assad’s effort to drown out pro-democracy protests exploded into clashes between government supporters and opponents Tuesday. Security forces opened fire and killed seven people, including a teenager, activists said.

It was the latest deadly turn in a 3-month-old uprising that appears unbowed by a relentless government crackdown. The violence flared a day after a speech in which Assad, trying to contain the situation, offered a vague promise of reform, one brushed off as too little, too late, by the opposition, which wants an end to the Assad family’s 40-year authoritarian rule.

In an attempt to blunt the uprising’s momentum, tens of thousands of regime supporters converged on squares in several major cities on Tuesday, shouting, “The people want Bashar Assad” and releasing black, white and red balloons — the colors of the Syrian flag.

They soon clashed with opposition supporters, drawing in security forces. In a main square in the central city of Hama, security personnel opened fire on anti-regime protesters, killing a 13-year-old boy, said the Local Coordinating Committees, which track the Syrian protest movement.

Three other people were reported killed in Homs, in central Syria, and three in the Mayadin district in the eastern city of Deir el Zour during pro- and anti-regime demonstrations.

The pro- and anti-Assad sides have fought each other in the past, but Tuesday’s bloodshed appeared to be the worst such violence.

“We are seeing an escalation by authorities today,” said Omar Idilbi, spokesman for the committees. “They are sending pro-government thugs along with security forces to attack protesters.”

The opposition estimates more than 1,400 Syrians have been killed and 10,000 detained after Assad unleashed his military and security forces to crush the protest movement, which sprang to life in March following the revolutions in Tunisia and Egypt.

The unending government repression and bloodshed appeared to be driving even Russia, a longtime Syria backer, to distance itself from Assad, as international pressure mounts for him to accept major political change.

“We need to apply pressure on the leadership of any country where massive unrest, and especially bloodshed, is happening,” Russian Prime Minister Vladimir Putin said in Paris.

“In the modern world it is impossible to use political instruments of 40 years ago,” Putin said of the Syrian’s tactics. It remained to be seen, however, whether this signaled a change in Moscow’s opposition to tough United Nations action on Syria.