OTTAWA Canadian fighter jets will be patrolling the edge of Russian airspace next week as part of NATO's response to the unravelling situation in Ukraine.

At least four of the six CF-18s sent overseas by the Harper government earlier this year have arrived at Siauliai Air Base in Lithuania, where they will fly air policing missions over the Baltic states.

The missions are meant to reassure European allies unnerved by Russia's seizure of the Crimean Peninsula, as well as the military action currently unfolding in eastern Ukraine.

The jets, which had been based in Romania on a training exercise, are tasked with defending Estonian, Latvian and Lithuanian airspace because those countries are unable to do so on their own.

The deployment comes against the unfolding backdrop of heavy fighting and an apparent Russian invasion, and brings Canadian fighters as close to Russian territory as they've been since the crisis began.

Foreign Affairs Minister John Baird says Russia's recent escalation of the fighting is totally unacceptable, and is calling on President Vladimir Putin's regime to halt its invasion.

Asserting that Russian soldiers and armaments had crossed into Ukraine to support the separatists, President Petro Poroshenko of Ukraine cancelled a trip to Turkey on Thursday and his national security council ordered mandatory conscription for the armed forces.

"Columns of heavy artillery, huge loads of arms and regular Russian servicemen came to the territory of Ukraine from Russia through the uncontrolled border area," Poroshenko said at the beginning of an emergency meeting of the Ukrainian Nation Security and Defense Council in Kiev.

Mercenaries, along with regular servicemen, were trying to overrun positions held by the Ukrainian military, he said, according to a statement on his official website.

"The situation is certainly extremely difficult and nobody is going to simplify it," Poroshenko said. "Still, it is controlled enough for us to refrain from panic." Panic could be used as a weapon just like tanks, armoured personnel carriers and rifles, he said.

Nonetheless, the deputy head of the council, Mykhailo Koval, announced after the meeting had concluded that mandatory conscription, which was suspended last year, would be restarted this fall, news services reported.

Poroshenko spoke as NATO released satellite images to corroborate its accusations that Russian forces were actively involved in the Ukrainian fighting. NATO also said that more than 1,000 Russian soldiers had joined the separatists battling the Ukrainian military.

Colonel Andriy Lysenko, a spokesperson for the national security council, said the Ukrainian military was planning a counteroffensive against the separatists and what he called "more and more Russians" in the country, but he declined to provide details about military plans. He also accused Russia of sending new anti-aircraft defence systems into eastern Ukraine.

Separatists aided by Russia held the town of Novoazovsk, he said, with Ukrainian forces having retreated a day earlier.

"There was a regrouping of our troops to better protect Mariupol," Lysenko said at a briefing in Kiev, saying the aim was to try to prevent the other side from penetrating the key southern city.

Russia officials continued to deny sending soldiers or weapons to Ukraine.

But the leader of the main separatist group in southeastern Ukraine said up to 4,000 Russians, including active-duty soldiers currently on leave, had been fighting against Ukrainian government forces, Russian television reported.

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"There are active soldiers fighting among us who preferred to spend their vacation not on the beach, but with us, among their brothers, who are fighting for their freedom," Alexander Zakharchenko, a rebel commander and the prime minister of the self-declared Donetsk People's Republic, said in an interview on Russian state-run television.

Zakharchenko said between 3,000 and 4,000 Russians had fought in the separatist ranks since the conflict erupted in the spring.