BEIRUT - Syrian troops and tanks battled rebels on Friday, opposition activists said, only four days before a troop pullback agreed by President Bashar Assad as part of international envoy Kofi Annan's plan to end a year of bloodshed.The renewed violence erupted a day after UN 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.UN-Arab League mediator Annan said "more far-reaching action is urgently required" to halt all forms of violence by April 12, following a withdrawal of the military's heavy weapons and troops from cities two days earlier.But activists reported tank fire in at least three urban centers on Friday -- the town of Douma near Damascus, the restive city of Homs and Rastan, north of Homs."Tanks went into Douma last night, then they left. Today at 7 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.The British-based Syrian Observatory for Human Rights said troops were fighting Free Syrian Army rebels in Douma as well as in Rastan on the highway between Homs and Hama cities.There were no immediate reports of casualties.Army shelling of villages in the northwestern province of Idlib has prompted a swelling exodus of Syrian refugees. Turkish Foreign Minister Ahmet Davutoglu said that in the past 24 hours, approximately 2,500 people had crossed into Turkey. Another official in Ankara placed the number at over 2,800.In a phone call to UN Secretary-General Ban Ki-moon early Friday, Davutoglu said that the Syrian military is using helicopters and stressed that the situation was urgent. Davutoglu also invited UN officials to monitor developments from the Turkish border.