Secretary of State Hillary Rodham Clinton said on Tuesday that Syrian insurgents fighting to topple President Bashar Assad were making territorial gains that would eventually become havens for "further actions by the opposition."

Clinton also said the United States, which has pledged to provide nonlethal aid to the Syrian insurgency, would work closely with the rebels, an indication that the United States might take a more active role as they secure swaths of Syrian territory. She also reiterated her call on Assad to start a political transition and vacate power.

Clinton's remarks, made in Washington, strongly suggested that the United States sees rebel momentum increasing in the 17-month-old uprising against Assad. She spoke as new evidence of fracturing in his government emerged, with the defection of its ambassador to Cyprus, Lamia al-Hariri, reported by Arab satellite channels. Al-Hariri's defection would make her the second ambassador to abandon Assad in the past two weeks. The first was his ambassador to Iraq.

The appearance of a shift in the momentum favoring the insurgents first emerged last week, when they assassinated key Assad security leaders with a bombing in the heart of Damascus. They also tied down Syrian forces in the capital, seized two border posts on the frontier with Iraq and have been fighting the army for days in Aleppo, Syria's second-largest city and a support base for Assad.

"We have to work closely with the opposition because more and more territory is being taken and it will, eventually, result in a safe haven inside Syria, which will then provide a base for further actions by the opposition," Clinton told reporters.

There were conflicting accounts on Tuesday of the fighting in Aleppo. A BBC correspondent, Ian Pannell, said in a Twitter post that Syrian forces had bombed Aleppo's eastern areas with fighter jets. If confirmed, the use of warplanes would signify an escalation by the Syrian government in its effort to crush armed resistance. But Pannell did not say whether he saw the aircraft, and others in Aleppo did not corroborate his assertion.

Fadi Salem, an Aleppo resident reached by telephone from Beirut, reported helicopters but no fighter jets. "I would say if there was one I would have heard it," said Salem, an academic.

Unarmed U.N. monitors, whose operations were suspended more than a month ago because of the combat dangers, have observed Syrian helicopter gunships in fighting with Syrian rebels but not the use of warplanes. The fighting in Aleppo, which entered its sixth day on Tuesday, is considered significant because of the city's allegiance to Assad's ruling Alawite sect. Analysts have said Aleppo's loss would be a significant victory for the insurgents, who are mostly members of Syria's Sunni majority.

Salem said families with children stood on street corners on Tuesday morning, pointing at the many helicopters circulating and occasionally firing in the distance. But as the sound of fighting widened to other neighborhoods around the central, old part of Aleppo, residents gradually withdrew indoors and life ground to a halt. The old city was completely shut down.

"People are still in shock that this is happening," Salem said. "They thought it would be limited to one neighborhood, but it is growing in size to other neighborhoods."