A Ukrainian military transport aircraft has been shot down in the east, amid fighting with pro-Russian separatist rebels, Ukrainian officials say.
They say the An-26 plane was hit at an altitude of 6,500m (21,325ft).
The plane was targeted with "a more powerful missile" than a shoulder-carried missile, "probably fired" from Russia. The crew survived, reports say.
Russia has made no comment. Separately, Nato reported a Russian troop build-up near the Ukraine border.
A Nato official confirmed to the BBC that the alliance had observed a significant increase of Russian troops, bringing their number to up to 12,000.
Russia denies supporting and arming the separatists, and has invited officials from the Organization for Security and Co-operation in Europe (OSCE) to monitor its border with Ukraine.
Direct talks plea
A statement on Ukraine President Petro Poroshenko's website said the An-26 was taking part in the "anti-terror operation" in the region.
Ukrainian Defence Minister Valeriy Heletei said a search-and-rescue operation was under way to locate missing crew members.
Andriy Lysenko, a spokesman for Ukraine's security and defence council, was later quoted by Ukrainian media as saying that eight people had been on board the plane.
In a Facebook message, some of those taking part in Ukraine's "anti-terror operation" said they knew "about the fate of two of the crew" and were checking information about the others.
Rebel forces - who earlier said they had targeted the aircraft in the Luhansk region - claimed they had captured the crew and were questioning them in the town of Krasnodon, reports in Russian media say.
Last month, the rebels shot down a Ukrainian Il-76 military transport plane as it was about to land at Luhansk airport, killing all 49 soldiers and crew on board.
Earlier on Monday, the Ukrainian air force said it had delivered "five powerful air strikes" in the region in an effort to end the blockade of the strategic airport in the rebel-held city.
Ukraine's military later said the airport had been "unblocked" and the army had retaken several villages.
Some air strikes hit the city on Monday, a resident of Luhansk told the BBC on Monday.
Meanwhile, the rebels claimed they had destroyed a Ukrainian armed convoy near the airport.
Fighting in the area has intensified since a rebel rocket attack near the Russian border on Friday, in which at least 19 government soldiers were killed.
President Poroshenko has vowed retaliation for that attack. On Monday, he also said Russian military officers were fighting alongside the separatists - a claim denied by Russia.
Tensions rose further over the weekend when Russia accused Ukrainian forces of shelling across the border, killing one person and wounding two others.
At least 15 civilians were killed in Luhansk and the neighbouring Donetsk region on Sunday, reports say.
Germany and Russia have urged direct talks between Kiev and the rebels.
And the UK and US have renewed calls for Russia to de-escalate the situation in eastern Ukraine.
Prime Minister David Cameron and President Barack Obama stressed the need for Moscow to take further steps towards peace or face further sanctions.
Separatist rebels have been fighting the government in Kiev since declaring independence in Luhansk and the neighbouring region of Donetsk in April.
The government began an "anti-terrorist operation" in April to crush the rebellion in the eastern regions.
More than 1,000 civilians and combatants are believed to have died in the fighting, which followed Russia's annexation of Crimea in March.