LONDON — Secretary of State John Kerry said Saturday that Russia's movement of tactical aircraft and surface-to-air missiles to Syria could pose a threat to American and allied forces, and made clear that the United States 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. Is Assad prepared to negotiate, really negotiate? Is Russia prepared to bring him to the table and actually find the solution to this violence?'' Kerry said after meeting with British Foreign Secretary Philip Hammond.
US officials said Russia sent a small number of fighter jets to a base in Syria on Friday, hours after Defense Secretary Ashton Carter talked with Russia Defense 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. The Russians have deployed at least one such system, according to an American official who was not authorized 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.
While the militants lack 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 spoke on condition of anonymity.
Russia's military moves in Syria are its first major 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.
The goal, Kerry said, is to make sure ''there's no potential of a mistake or of an accident of some kind that produces a greater potential of conflict.''
While Kerry stressed the United States demand that Assad must go, he stated explicitly that an acceptable resolution to the Syrian war would allow Assad to remain in place for a limited period.
''We're not being doctrinaire about the specific date or time,'' Kerry said. ''But right now, Assad has refused to have a serious discussion and Russia has refused to bring him to the table in order to do that. So that's why we're where we are.''
As for an Assad timetable, Kerry said, ''it doesn't have to be on day one or month one or whatever,'' but in context of negotiations.
Kerry also said Washington considers Assad ''a magnet'' for foreign fighters who are filling the Islamic State group's ranks.
''So there's a lack of logic,'' Kerry said, for the Russians to say ''they are bringing in more equipment, shoring up Assad, at the same time they say they're going after'' militants.
In the meantime, a Syrian rebel group said it fired rockets at a coastal air base said to be used by Russian troops.
In a video posted Friday, members of the Islam Army warn the Russians that they will not enjoy peace in Syria. The fighters are then seen launching multiple rockets from a mountainous area.