BELFAST, Northern Ireland - A free Syria can never include the leader now fighting to hang on to his three-decade regime, U.S. Secretary of State Hillary Rodham Clinton said Friday, underscoring the American goal for new U.S.-Russian cooperation on Syria.

Clinton called talks she held Thursday with the Russian foreign minister constructive but very preliminary. The two diplomats agreed to support efforts by the United Nations-Arab League envoy for Syria, a decision Clinton said was influenced by rapid escalation of the 20-month civil war.

"I don't think anyone believes that there was some great breakthrough," Clinton said. "No one should have any illusions about how hard this remains."

Anyone with any influence on the regime or the rebels is duty-bound to try to intervene for a "political transition," in Syria, Clinton said.

The transition phrase is a euphemism the United States has employed to make the goal of ousting Syrian President Bashar al-Assad easier for his Russian backers to swallow. But Clinton followed it with a flat statement that Assad cannot hope to survive any reordering of the Syrian government that the new cooperation might foster.

The United States wants freedom and protections for all Syrians and will hold all parties to account, Clinton said.

"A future of this kind cannot possibly include Assad," Clinton said.

She spoke in Northern Ireland, whose struggle to overcome decades of political violence she often invokes when talking of newer conflicts. In that region, a political row over how many days the Union flag flies over a government building has set off new demonstrations and more dangerous reminders of the bombing campaigns of the recent past. Hours before Clinton came to Belfast, police found two bombs in the region.

In Syria, meanwhile, rebels fighting to topple Assad set their sights Friday on the capital's international airport in a bid to cut off the regime's supplies, clashing with government troops nearby and again forcing the closure of the airport road.

A fighter who is part of the push against Damascus International Airport declared it a legitimate target, claiming that the regime has stationed troops and elite forces there as well as military planes that transport ammunition.

Losing control of the airport would be a major blow to the regime, which has recently lost two air bases near the capital.

It was unclear just how close to the airport, a few miles south of the capital, the battles reached. Fighting has intensified in the past week in the southern districts of the Syrian capital and its suburbs.