A Ukrainian military transport aircraft has been shot down in the east, amid fighting with 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.
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 military alliance had observed a significant increase of Russian troops bringing their number to an estimated 10-12,000.
Direct talks plea A statement on Ukraine President Petro Poroshenko's website said the An-26 was taking part in an "anti-terror operation" in the region.
The statement quoted Ukrainian Defence Minister Valeriy Heletei as saying that a search-and-rescue operation was now under way.
It was not immediately known if there were any other people on board the aircraft. Rebel forces earlier said they had targeted the aircraft in the Luhansk region.
This comes as the Ukrainian air force said it had delivered "five powerful air strikes" on targets in the region in a bid to end a rebel blockade of a strategic airport there.
Several targets were hit near the airport in the rebel-held city of Luhansk, a military spokesman said.
Ukraine's military later said the airport had been "unblocked" and the army had retaken several villages.
A resident in Luhansk has told the BBC that some air strikes hit Luhansk 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.
Ukrainian President Petro Poroshenko has vowed tough retaliation for that attack.
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 where it is accused of backing the rebels.
In a telephone conversation, 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.