LUGANSK, Ukraine (AFP) - 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 Molotov 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 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.
Mr 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".
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.