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.

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.

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 kilometers (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.

Ukraine's Western-backed President Petro Poroshenko vowed to deal the rebels "an adequate response" after the attack and signaled 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 tricolor with the help of a wooden pole.

Later a Molotov cocktail hit the wall of the building, but it was quickly extinguished.

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".