Lithuania says its honorary consul in the rebel-held city of Luhansk in eastern Ukraine has been murdered by "terrorists" there.
Lithuania's Foreign Minister Linas Linkevicius tweeted with "deep sorrow" that "Mr Mykola Zelenec was kidnapped & brutally killed by terrorists there".
Ukraine routinely calls the pro-Russian separatists in Luhansk "terrorists".
The news came amid reports that some Russian aid lorries had reached Luhansk without any permission from Ukraine.
There has been no comment from the rebels yet on the Lithuanian diplomat's death.
Lithuania is among the most vociferous EU member states in its criticism of Russian actions in Ukraine. The EU and US accuse Russia of fomenting the separatist rebellion in eastern Ukraine.
Mr Linkevicius described the entry of the Russian aid convoy into eastern Ukraine as "a blatant violation of international law", echoing Ukraine's condemnation of the move.
The UN Security Council is holding an emergency session at Lithuania's request to discuss the issue.
The Ukraine crisis has heightened tensions between Russia and the three Baltic republics - including Lithuania - which used to be Soviet republics governed from Moscow.
Russia's ambassador to the United Nations, Vitaly Churkin, lambasted the Lithuanian delegation on Friday.
He scorned "the indefatigable delegation of Lithuania, which is always torpedoing all productive, constructive initiatives we've had in the Security Council".
He said Lithuania had amended a Russian proposal calling for a ceasefire while the aid was distributed in Luhansk. He said the Lithuanian delegation "sent in amendments where they dropped the reference to Russia and included a reference to the European Union, and then dropped the reference to a ceasefire".
At the UN, he added, "the Lithuanian delegation starts working, and of course we know the division of labour - the US and UK are not far behind".