Pro-Russia separatists shot down a Ukrainian military transport plane over Lugansk on Saturday, killing all 49 crew and troops on board.

It marked a significant setback for Kiev’s forces, which are struggling to suppress an armed revolt by opponents of new President Petro Poroshenko’s government.

Mr Poroshenko announced a day of mourning following the shooting down and vowed to deal the rebels “an adequate response,” signalling an imminent intensification of the current offensive.

In other fighting, five border guards were killed and seven wounded on Saturday in the southern port of Mariupol when their column of vehicles was ambushed.

Meanwhile, Russia accused Ukrainian police of doing nothing to stop an attack on the Russian embassy in Kiev.

Several diplomatic cars were overturned and damaged by protesters who also threw paint, eggs and stones at the embassy building as a demonstration over the attack turned into a riot.

Russia condemned Kiev police “inaction” as “a grave violation of Ukraine’s international obligations.”

Washington also delivered Kiev an unusual rebuke by urging “authorities to meet their Vienna Convention obligations to provide adequate security.”

But the US followed up by accusing Russia of helping the rebels by sending tanks and rocket launchers — a charge the Kremlin denied.

Pro-Russian fighters insist they get their weapons from captured military stockpiles.

The US and the European Union have imposed sanctions on Russia — largely limiting them to visa bans and asset freezes on individuals, banks and companies — but say they could get tougher.