Russia's domestic intelligence agency, the Federal Security Bureau (FSB), has accused five detained Crimean Tatar activists of sabotaging a gas pipeline one day after Ukraine dismissed the charges as fabricated.
The FSB claimed on September 7 that Ukrainian military intelligence procured an explosive device and promised a cash reward of $2,000 to the men to plant it.
All five men -- including Nariman Dzhelyal, deputy chairman of the Crimean Tatars' self-governing assembly, the Mejlis, which was banned in Crimea after Russia's seizure of the peninsula in 2014 -- were arrested over the weekend.
Ukrainian Deputy Foreign Minister Emine Dzheppar on September 6 accused Russian investigators of fabricating charges against those arrested and mistreating them.
The five men could face a lengthy prison sentence if found guilty.
The gas pipeline, near the Crimean capital, Simferopol, was damaged in August.
Ukrainian President Volodymyr Zelenskiy said the arrests were in retaliation for the inaugural summit of the Crimean Platform, an international initiative to bring about the restoration of Ukrainian-Russian relations by ending the Russian occupation of Crimea.
EU foreign affairs spokesman Peter Stano said the bloc "considers the detentions to be politically motivated and illegal under international law," and call for the detainees' release.
Since Russia occupied Crimea in 2014, Russian authorities have prosecuted dozens of Crimean Tatars for allegedly belonging to the Hizb ut-Tahrir Islamist group that is banned in Russia but not in Ukraine.
Rights groups and Western governments have denounced what they describe as a campaign of repression in Crimea against members of the Turkic-speaking Crimean Tatar community and others who have spoken out against Moscow's takeover of the peninsula.
Russia has backed separatists in a war against Ukrainian government forces that has killed more than 13,200 people in eastern Ukraine since April 2014.