BEIRUT - Syria's rebels stepped up attacks on strategic sites including a sprawling military complex in the country's north on Friday, while reports emerged that President Bashar al-Assad's forces continued to fire Scud missiles at rebel areas.

Russian President Vladimir V. Putin told European leaders that Russia does not seek to protect Assad but that only a negotiated solution can end the conflict - an outcome that looks unlikely as rebels make gains across the country.

While few observers expect Syria's 21-month-old conflict to end soon, most say steady rebel advances appear to be tipping the balance in favor of those fighting to topple Assad's regime.

Anti-regime activists reported rebel attacks on strategic government sites in northern Syria on Friday, showing rebel efforts to cut government supply lines, free up roads and seize arms from government bases.

Near the northern city of Aleppo, rebels clashed with government forces at a sprawling military complex by the town of Al-Safira, activists said.

An activist in the town said the complex contains military factories and a scientific research center that produces chemical weapons as well as an air-defense and an artillery base.

Rebels seized the artillery base earlier this week and have been trying to seize the air-defense base, said Hussein, who gave only his first name for fear of retribution.

The town's proximity to the facilities has cost it dearly, he said. Government air strikes often target civilian areas, and more than two-thirds of the town's residents have fled, fearing the regime will use chemical weapons.

On Friday, Russia's Putin said that his country does not seek to preserve Assad's rule but wants a "democratic regime in Syria based on the expression of people's will" - an outcome he said can only come through negotiations.

"We aren't a defender of the current Syrian leadership," Putin told European leaders in Brussels.

Throughout the conflict, Russia has been one of Assad's greatest backers, selling him arms and, along with China, protecting Syria from censure by the U.N. Security Council.

But Russian officials have recently distanced themselves from Assad's regime, suggesting they are resigned to his potential ouster.

Also Friday, NATO's top official said that Syria has continued to target rebel areas with Scud-type missiles.

Secretary-General Anders Fogh Rasmusen called the attacks "acts of a desperate regime approaching collapse."