Syrian troops and tanks battled rebels on Friday, opposition activists said, only four days before a troop pullback agreed to by President Bashar al-Assad as part of international envoy Kofi Annan's plan to end a year of bloodshed.

The renewed violence erupted a day after U.N. Secretary-General Ban Ki-moon said the conflict was worsening and attacks on civilian areas persisted, despite assurances from Damascus that its troops had begun withdrawing under the plan.

Annan said both the government and opposition must stop fighting at 6 a.m. Syrian time on April 12, if Damascus meets its deadline 48 hours earlier to pull back troops from cities and cease heavy weapons use in populated areas.

But activists reported tank fire in at least three urban centers on Friday - the town of Douma near Damascus, the restive city ofHoms and Rastan, north of Homs.

"Tanks went into Douma last night, then they left. Today at 7:00 in the morning they came back. There has been shelling on Douma since the morning. We are not sure if people were killed but the shelling did not stop," a local activist said.

"At least 5 tanks and 10 buses loaded with security men and Shabiha (pro-Assad militia) entered Douma," he said.

In Rastan, "the tanks began advancing in the morning, then the fighters from the Free Army went out and confronted them," said one activist. "They blocked the advance and the Assad army left. Then artillery started."

Accounts of the violence are difficult to verify because Syria's government restricts access to independent journalists.

Assad blames the unrest on foreign-backed "terrorists" and has put forward a reform program which includes holding a parliamentary election on May 7.

His opponents have dismissed Assad's reform pledges as an attempt to play for time, saying it is impossible to have a valid election amid continuing repression.