Russian military aircraft were buzzing Canadian frigate HMCS Toronto which is on a U.S.-Ukraine training exercise in the Black Sea – low-level flyovers that signal Russia is watching them as tensions remain high between Moscow and Kiev.

"While the Russian military aircraft that circled the HMCS Toronto did not in any way pose a threat to the Canadian ship, their actions were unnecessarily provocative and risk escalating tensions even further," Defence Minister Rob Nicholson said.

Russian news agency Itar-Tass quoted General-Major Igor Konashenkov, a senior Russian Defence Ministry official, as saying two Russian military aircraft - an SU-24 fighter jet and an AN-26 transport plane - had flown through the area on a routine flight but denied any provocative behaviour.

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"The planes' flight path passed through the area where the Canadian naval ship Toronto was, but without making an approach on the foreign military vessel," he said.

But sources say there were three Russian aircrafts that flew low over HMCS Toronto on Sunday.

"We expect this," the military source said. "They're going to play cat and mouse with us for as long as we're there."

At least one of the Russian planes came within 300 metres of HMCS Toronto, a government source said.

The flyover is hardly a hostile action but nevertheless sends a message that Moscow has taken notice of the training exercise in the same body of water that borders Crimea and contains Russia's Black Sea fleet. Russia annexed Ukraine's Crimean peninsula this spring and has been accused of working to destabilize and break up its smaller neighbour in recent months, including sending 3,000 soldiers into eastern Ukraine to fight with pro-Kremlin rebels against Kiev's forces.

HMCS Toronto, which is part of NATO's standing maritime group, entered the Black Sea September 6 as part of the Operation Sea Breeze exercise that runs Sept. 8 to 10.

HMCS Toronto has been deployed to the Mediterranean for months as part of Canada's contribution to the NATO reassurance mission.

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Operation Sea Breeze, although not a NATO-run exercise, brings together American and Ukrainian vessels as well as ships from France, Spain, Romania, Turkey and Bulgaria.

HMCS Toronto will also conduct port visits in the Black Sea to "enhance diplomatic and defence relations with strategic partner," the Canadian government has said.

Other vessels in the exercise include the U.S. Navy guided-missile destroyer, the USS Ross, the Spanish frigate ESPS Almirante Juan de Borbon and the Romanian frigate ROS Regele Ferdinand.

Under the Montreux Convention of 1936, warships of non-Black Sea states can stay in the Black Sea for no more than 21 days. The alliance has managed to secure its presence in the area by constantly rotating warships there.

- With a report from Reuters