Dozens of tanks have been shelling the town of Idlib. It's been one of the centers of the uprising against President Bashar Assad's rule.
On Saturday, Assad told Annan that a political solution is impossible as long as "terrorist groups" are trying to destabilize Syria. Many low-level soldiers in the area have joined the opposition and fight along with civilians who have taken up arms as part of the loosely organized Free Syrian Army.
Many fear the offensive in Idlib could end up like the regime's campaign against a rebel-held neighborhood in the central city of Homs. Troops besieged and shelled Baba Amr for weeks before capturing it on March 1. Activists say hundreds were killed, and a U.N. official who visited the area this week said she was "horrified" by the destruction in the district, now virtually deserted.
The U.N. estimates more than 7,500 people have been killed since the uprising began last year, but activists put the death toll at more than 8,000.
The Associated Press contributed to this story.