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A permanent Russian base in Syria would unnerve both the United States and Israel, and would be close to the strategic Turkish port of Ceyhan, the terminus of a major new oil pipeline linked to the Azerbaijani port city of Baku.

Washington has clashed repeatedly with Syrian President Bashar Assad over Iraq, Lebanon and other Middle Eastern hot spots.

But Russia’s military has long-standing links to its Syrian counterpart, dating back to the Cold War. About 2,000 Russian advisers reportedly are serving as trainers and advisers to the Syrian armed forces.

Moscow has agreed to write off more than 70 percent of an $11 billion debt owed by Syria, leading to speculation that the concession was granted in return for expanded rights at Tartus and Latakia.

Mr. Cohen said the Russian maneuvering reflected in part the strategic and political challenge of losing Sevastopol in 10 years.

“That would really be a blow to the prestige and morale of the Russian navy, which has been there for more than 200 years,” he said.