Russian President Vladimir Putin attends a wreath-laying ceremony at the Tomb of the Unknown Soldier in Alexander Garden by the Kremlin Wall to mark Defender of the Fatherland Day, in Moscow. Putin thanked the servicemen for the dignified performance of their duty and noted the combat effectiveness of the armed forces. He also expressed special gratitude to the veterans who “taught us to win and not give up.” Photo: VCG

 Russia on Saturday said it would expel a Ukrainian diplomat, prompting an immediate pledge of retaliation from Kiev, further escalating tensions over Moscow's troop buildup on Ukraine's eastern flank.

The detention of a Ukrainian consul in the second city Saint Petersburg comes at a time of global concern of a repeat of major conflicts in 2014.

Moscow claimed the Ukrainian diplomat had been caught red-handed trying to obtain sensitive information. In response, the Ukrainian foreign ministry protested "the illegal" detention of its diplomat and said Kiev would expel a senior Russian diplomat.

Kiev has been battling rebels in eastern Ukraine since 2014 and clashes intensified early this year, effectively shredding a ceasefire agreed in July 2020.

Around 30 Ukrainian troops have been killed since the start of the year, compared to 50 last year. Most of them were victims of sniper fire. 

"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. Kiev said the diplomat had spent several hours in detention.

The Russian foreign ministry said it had summoned Ukraine's charge d'affaires Vasyl Pokotylo and told him that the Ukrainian diplomat had 72 hours to leave the country beginning Monday.

"The Russian side pointed to the inadmissibility of such kind of activity," the foreign 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 toward the Russian Federation," the FSB said.

In Kiev, the Ukrainian foreign ministry said it protested the diplomat's detention and rejected Russia's accusations.

Tensions between Russia and Ukraine come against the background of a new war of words between Moscow and Washington as new US President Joe Biden seeks a tougher line against Putin.

Ukraine is not the only diplomatic flashpoint between Russia and the West.

On Saturday, the Czech authorities announced it would be expelling 18 Russian diplomats identified by local intelligence as secret agents of the Russian SVR and GRU services.