Russia on Saturday said it would expel a Ukrainian diplomat, prompting an immediate pledge of retaliation from Kyiv, further escalating tensions after Moscow’s troop buildup on Ukraine’s eastern flank. The detention of a Ukrainian consul in the second city, St Petersburg, comes at a time of global concern over a possible repeat of Moscow’s 2014 aggression, when Russia annexed the peninsula of Crimea and backed separatists in Ukraine’s east. Moscow claimed that the diplomat had been caught “red-handed” trying to obtain sensitive information. In response, the Ukrainian foreign ministry condemned “the illegal” detention of its diplomat and said Kyiv would expel a senior Russian diplomat in response. Kyiv has been battling Russian-backed separatists in eastern Ukraine since 2014 and the clashes there intensified early this year, effectively shredding a ceasefire agreed last July. About 30 Ukrainian troops have been killed since the start of the year, compared with 50 in all of 2020. Most were victims of sniper fire. Russia has detained a number of Ukrainian nationals on suspicion of spying in recent years, but it is rare for diplomats to be arrested. “A Ukrainian diplomat, a consul of Ukraine’s consulate general in Saint Petersburg, Alexander Sosonyuk, has been detained by Russia’s FSB,” the security service said in a statement, using the Russian-language spelling of his name. He was held on Friday, the security service said. Kyiv said the diplomat had spent several hours in detention. The Russian foreign ministry said on Saturday it had summoned Ukraine’s chargé d’affaires, Vasyl Pokotylo, and told him that the Ukrainian diplomat had 72 hours to leave the country from 19 April. “The Russian side pointed to the inadmissibility of such kind of activity,” the ministry said in a statement. Russia’s domestic intelligence agency said Sosonyuk had been caught “red-handed” during a meeting with a Russian national as he sought to receive “classified” information. “Such activity is not compatible with his diplomatic status and is clearly hostile in nature towards the Russian Federation,” the FSB said. “In conformity with international law, measures will be taken against the foreign diplomat.” In Kyiv, the Ukrainian foreign ministry said it contested the diplomat’s detention and rejected Russia’s accusations. Spokesman Oleg Nikolenko said Russia had “crudely” violated diplomatic conventions and sought to escalate tensions. “In response to the abovementioned provocation, a senior diplomat of the Russian embassy in Kyiv must leave Ukrainian territory within 72 hours beginning 19 April,” Nikolenko told AFP. He did not provide further details. Faced with the largest deployment of Russian troops on Ukraine’s borders since 2014, the president, Volodymyr Zelenskiy, has requested more help from the west, and western leaders have urged the Russian president, Vladimir Putin, to stop intimidating his neighbour. Tensions between Russia and Ukraine come against the background of a new war of words between Moscow and Washington as the new US president, Joe Biden, seeks to take a tougher line against Putin. The US on Thursday announced sanctions and the expulsion of 10 Russian diplomats in retaliation for what the White House says is the Kremlin’s US election interference, a massive cyber-attack, and other hostile activity. Russia said on Friday that it would expel US diplomats and sanction US officials in response, while recommending that the US envoy leave Russia “for consultations”.