US Secretary of State John Kerry says Russia's movement of tactical aircraft and surface-to-air missiles to Syria could pose a threat to American and allied forces, while making clear the US could accept a resolution to the civil war that allows President Bashar Assad to remain in power for an unspecified time.
"We're prepared to negotiate," Kerry told reporters after meeting with British Foreign Secretary Philip Hammond in London.
"(But) is Assad prepared to negotiate, really negotiate? Is Russia prepared to bring him to the table and actually find the solution to this violence?"
US officials said Russia sent a small number of fighter jets to a base in Syria on Friday, hours after US Defense Secretary Ash Carter talked with Russia Defence Minister Sergei Shoigu in the first military contacts between the two countries in some time.
"Clearly, the presence of aircraft with air-to-air combat capacity ... and surface-to-air missiles raise serious questions," Kerry said.
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The Russians have deployed at least one such system, according to an American official, who was not authorised to discuss military matters and spoke on condition of anonymity.
Russia says its recent military buildup in Syria is designed to fight the Islamic State group.
While IS lacks an air force, the Russian aircraft are capable of striking ground targets and providing close air support for ground forces, a US intelligence official said.
The official was not authorised to discuss military matters and spoke on condition of anonymity.
Russia's military moves in Syria are its first major expeditionary force deployment outside the former Soviet Union since the war in Afghanistan, the official said.
Kerry said the military-to-military talks with the Russians are designed to make sure there are no incidents between Russian and American forces.
The discussions also amount to a tacit acceptance of the Russian buildup, after weeks of warnings from Washington against any Russian escalation in Syria.
