The United States is disturbed by Russia's movement of tactical aircraft to Syria, U.S. Secretary of State John Kerry said Saturday, acknowledging that the jets could pose a threat to American and allied military forces.

U.S. officials say Russia moved a small number of fighter jets to a base in Syria on Friday, hours after U.S. Defense Secretary Ash 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 ... raises serious questions," Kerry said, responding to a question after meeting with British Foreign Minister Philip Hammond.

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, according to a U.S. intelligence official who was not authorized 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.

In another apparent concession, Kerry stated explicitly that the U.S. could accept a resolution to the Syrian war that allowed President Bashar Assad to remain in place for a time before departing, as the U.S. long has wanted.

"We're not being doctrinaire about the specific date or time — we're open," Kerry said, adding that Assad doesn't have to leave "on day one, or month one, or whatever."

He later added that the U.S. considered Assad a magnet for the 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 to shore up Assad at the same time they say they are going after" the militants.

Meantime, a Syrian rebel group claims 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 are seen loading and launching multiple rockets from a mountainous area.

Kerry and Hammond said they also discussed the situations in Yemen, Libya and Ukraine. Kerry also urged restraint in response to days of clashes around the Jerusalem holy site known to Muslims as the Noble Sanctuary and to Jews as the Temple Mount.

"All of us join together in urging everybody to keep the calm," Kerry said.