Lugansk: Ukraine marks a national day of mourning Sunday, vowing to retaliate after pro-Kremlin rebels downed a military plane killing 49 personnel in their deadliest single attack against government forces in the east of the country.
Russia and Ukraine also meet for key gas talks Sunday to avert a cut in Russian supplies that would affect large swathes of Europe.
The new round of talks comes a day after an irate mob smashed the Russian embassy's windows in Kiev and threw a cocktail against its walls, in the wake of the attack that brought down the transport plane in Ukraine's restive east.
The United States accused Russia of helping the insurgency by sending tanks and rocket launchers to the pro-Moscow rebels in the former Soviet republic -- a charge the Kremlin denied.
A commander in the rebel-held eastern city of Lugansk, where the plane was shot down, showed pieces of the Ilyushin-76 transporter's charred debris in a wheat field a dozen kilometres (around eight miles) outside the airport.
The man known to his unit as Mudzhakhed (Sacred Fighter) said the plane tried to dump fuel after the rebels hit its engines. The four-engine transporter crashed on its second landing approach after being hit by heavy machine gun fire.
He listed the mostly Russian-speaking region's grievances against the new more nationalist leaders in Kiev.
"They brought machine guns and ammunition," Mudzhakhed said. "We do not like people telling us what to do."
Ukraine's Western-backed President Petro Poroshenko vowed to deal the rebels "an adequate response" after the attack and signalled an imminent intensification of an offensive being waged against the insurgents. He proclaimed Sunday a national day of mourning.
Poroshenko spoke moments before a crowd of several hundred smashed windows in the Russian embassy building and overturned luxury cars belonging to its staff before pulling down its tricolour with the help of a wooden pole.
Later a Molotov cocktail hit the wall of the building, but it was quickly extinguished, according to an AFP reporter on the scene. Russia condemned Kiev police's inaction as "a grave violation of Ukraine's international obligations".
Washington also delivered Kiev a rare rebuke by urging "authorities to meet their Vienna Convention obligations to provide adequate security".
- 'Ukraine no longer exists' -
German Chancellor Angel Merkel and French President Francois Hollande expressed "extreme concern" over Ukraine's spiralling violence in a joint phone conversation with Russia's Vladimir Putin, in which they said it was important to rapidly reach a ceasefire.
And British Foreign Secretary William Hague said "the international community stands ready to impose further sanctions if Russia continues to provoke instability in Ukraine".
Lugansk, an industrial hub of 400,000 inhabitants, has been under effective rebel control since the eastern uprising began in early April.
Nearby border guard units have come under brazen attacks by fighters from strife-torn Russian regions such as Dagestan and Chechnya.
On Saturday, Ukraine's federal forces suffered still more casualties when three border guards were killed and four wounded after being ambushed in the southeastern port of Mariupol -- captured with great fanfare by federal forces the day before.
The Ukrainian forces have so far managed to hold on to Lugansk's airport and use it to rotate equipment and troops serving in the campaign to quell the separatist unrest.
But they have had to repel an increasingly frequent series of raids by gunmen. At the end of May, the military seized back the main international airport in the southeastern city of Donetsk, which had been briefly overrun by the rebels.The eastern insurgency has now claimed at least 320 lives of civilians and fighters on both sides.
- Monday gas cut? -
Poroshenko's troubles have been compounded by the threat of Ukraine being cut off from economically vital Russian gas shipments as early as Monday because of the bitter gas price dispute, which talks Saturday failed to resolve.
"No solution was found, and the negotiations will continue Sunday morning," Ukrainian Energy Minister Yuriy Prodan told journalists after about two hours of EU-brokered talks in the Ukrainian capital.
Ukraine receives half its gas supplies from Russia and transports 15 percent of the fuel consumed in Europe. Moscow had nearly doubled the price it charges Kiev in the wake of the February ouster of a Kremlin-backed president.
Ukraine said heading into Saturday's urgent round of talks in Kiev that it was ready to make a $1.95 billion (1.45 billion euro) payment demanded by Moscow by Monday morning if Russia agreed to cut its ongoing price to $326 from $485.50 for 1,000 cubic metres of gas.But Russia had called a price of $385 per 1,000 cubic metres its final offer.
- 'Russian tanks' -
The United States on Friday accused Russia of secretly sending tanks and rocket launchers to the rebels in a bid to further destabilise its western neighbour.
US State Department spokeswoman Marie Harf raised the prospect of further Western economic sanctions if Russia failed "to demonstrate its commitment to peace".
And NATO released photographs Saturday of what it said were suspected Russian tanks in the restive region that "do not bear markings or camouflage paint like those used by the Ukrainian military".
It said the images raised "significant questions concerning Russia's role in facilitating instability in eastern Ukraine". In Bern, OSCE chief Didier Burkhalter expressed his deep concern about the escalation of tension and called on all those who may have an influence on the situation to act with utmost restraint.
Burkhalter, who is also Swiss foreign minister, condemned the downing of the transport plane and offered his sincere condolences to Poroshenko and the people of Ukraine.
He called on the Ukrainian authorities to take all necessary measures to provide adequate security for all diplomatic missions.