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A U.S. Navy surveillance aircraft in international airspace off the coast of Syria was intercepted three times by a Russian jet fighter over three hours on Tuesday, including a high-speed pass that was deemed unsafe and put the plane at risk, the U.S. Sixth Fleet said.

"While the Russian aircraft was operating in international airspace, this interaction was irresponsible," the Sixth Fleet said in a statement.

"We expect them to behave within international standards set to ensure safety and prevent incidents," it said.

Open gallery view The P-8's tracks, right along Syria's ATC boundary, 20 km off Russia's naval base in Tartus, yesterday. Pictured: At Perth Airport, 2014 Credit: Darren Koch

The Sixth Fleet said the first and third intercepts of the P-8A Poseidon were deemed to be safe but the second intercept involved a high-speed pass directly in front of the U.S. aircraft that produced wake turbulence and "put our pilots and crew at risk."

The intercept lasted about 28 minutes, it said.

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U.S. Navy and Air Force reconnaissance planes have stepped up their intelligence gathering sorties off the Syrian coast since Russia deployed its air force in late 2015 to help the Assad regime in its fight against the Islamic State and rebel groups.

The P-8A conducts anti-submarine and anti-surface warfare, surveillance and reconnaissance, specifically aimed the Russian naval facility in Syria's Tartus, Khmeimim air base in Latakia and missile defense systems.

The Air Force's SIGINT and ELINT Rivet Joint planes have also been tracked in the area on an almost daily basis.

Russia's Ministry of Defense said it had scrambled a Sukhoi Su-35 jet from its air base in Syria to intercept the U.S. plane which it said had been approaching Russia's Tartus naval facility on the Syrian coast, the RIA news agency reported on Wednesday.

Moscow denied its aircraft had acted irresponsibly, saying it had stayed at a safe distance and had returned to its base after the U.S. aircraft changed course.

Flight tracking software showed just how close the U.S. aircraft came to the Syrian ATC boundary, putting it roughly 20km from the Russian naval facility.