DAVYDO-MYKILSKE : Ukrainian forces hunted for the crew of a downed military plane Monday after Kiev said the aircraft was “likely” shot down from Russia, ratcheting up tensions along their volatile border.

“Crew members from an AN-26 plane... that was shot down have established contact with the general staff,” said a statement on the Ukrainian presidency website, without giving further details on the whereabouts of the eight people on board.

It said the transport aircraft had been flying too high to be hit by portable missile systems used by the rebels, meaning the shots had come “likely from the territory of the Russian Federation”.

A spokesman for Ukraine’s National Security and Defence Council said they thought the plane was hit either by a Greyhound surface-to-missile or a projectile fired from a jet that had taken off in Russia.

An AFP crew found the wreckage of the plane strewn around a field in the rebel-controlled eastern Lugansk region close to the border with Russia and local residents said it had come down shortly after midday with some parachutes spotted in the sky.

Ukrainian military spokesmen said they had been in touch with two crew members and an AFP journalist said that charred human remains were visible amid the detritus of the crash.

Rebels claimed their fighters had shot down the craft and told Russia’s Interfax news agency they had captured four crew members and were interrogating them.

Kiev’s accusation will ramp up nerves along the porous border between the two ex-Soviet neighbours - across which Kiev accuses Moscow of pouring fighters and weapons - with NATO accusing Russia of upping troop numbers on the frontier from less than 1,000 to as many as 12,000.

“This is not a step in the right direction. It is a step away from de-escalating the situation,” a NATO official said Monday.

Tensions had already soared after a shell reportedly from the Ukrainian side killed a Russian civilian on Sunday.

The foreign ministry in Moscow warned that Kiev risked “irreversible consequences” and a report in respected daily Kommersant cited a source close to the Kremlin as saying Russia was weighing up “targeted retaliatory strikes” against Ukrainian positions.

President Vladimir Putin’s spokesman Dmitry Peskov however dismissed the report, telling AFP: “I don’t comment on this in any way. It’s complete nonsense.”

Russia’s foreign ministry said Monday that it was inviting international observers from the Organisation for Security and Cooperation in Europe (OSCE) to monitor the border as a “goodwill gesture”.

Kiev has denied that its forces were behind the shelling and Western-backed President Petro Poroshenko called Sunday on the West to condemn “attacks by Russian soldiers of positions held by Ukrainian servicemen”, in a phone conversation with EU Council President Herman van Rompuy.

- Battle scars around rebel city -

The downing of the aircraft - the latest since a military plane was shot down last month killing all 49 on board - came as Ukrainian forces claimed to be making gains around the key rebel stronghold of Lugansk.

Poroshenko said government forces had managed to break through a blockade by pro-Moscow rebels to reach soldiers camped out at the strategic airport in the insurgent-held bastion.

An AFP journalist saw the burned-out wreckage of three unidentified armoured vehicles along the road to the airport and later heard explosions and plumes of black smoke close to the town Monday afternoon.

Lugansk is the capital of one of the rebels’ two self-declared “People’s Republics” and - along with million-strong Donetsk - now finds itself in the crosshairs of Kiev’s reinvigorated military push to quash the three-month insurgency tearing apart the ex-Soviet state.

Local authorities said three people were killed and 14 wounded in various incidents around the city over the last 24 hours, adding to a bloody weekend that saw one of the highest two-day civilian tolls so far in the conflict that has now claimed some 550 lives.

Poroshenko has previously vowed to kill “hundreds” of gunmen for every lost soldier and ordered an airtight military blockade of Lugansk and Donetsk.

European leaders responded by joining forces with Putin in a bid to persuade Poroshenko to put the brakes on a conflict first sparked by the February ouster of a Kremlin-backed president and fanned by Russia’s subsequent seizure of Crimea.

Hopes of a truce rested on a meeting between Putin and Poroshenko - the second since the Ukrainian president’s May election - that seemed on the cards on the sidelines of the World Cup final in Rio de Janeiro.

But Poroshenko pulled out at the last moment, leaving Putin to discuss the crisis with German Chancellor Angela Merkel.