Russia has accused Ukraine of abducting two of its servicemen from the Crimean peninsula, which it annexed in 2014.
The Russian defence ministry said the men were “illegally detained” on Sunday.
Russia called their arrest “another act of gross provocation” and called for their “immediate return”.
The men have been named as Ensign Maxim Evgenyevich Oditsov and Junior Sergeant Alexander Vyacheslavovich Baranov.
Russia said they were taken from Crimea to Ukraine’s Mykolaiv region.
The Security Service of Ukraine (SBU) said the detainees were apprehended after crossing into Ukrainian-controlled territory.
‘Confessions and sabotage’
In a statement, the Russian defence ministry accused Ukraine’s security agencies of “fabricating” criminal charges against the two soldiers “for crimes allegedly committed against Ukraine”.
It warned that “psychological and physical pressure” could be exerted on them to produce confessions.
Relations between Russia and Ukraine are at freezing point after Russia’s annexation of the Crimean Peninsula more than two years ago, and support for pro-Russian insurgents in eastern Ukraine.
Earlier this month, a number of Ukrainian citizens were arrested in Crimea and accused of plotting acts of sabotage against Russia.
Several men were shown on Russian TV confessing to the claims.
Ukrainian officials have rejected the accusations.