Russian marines, who guarded Russia's Hmeymim air base in Syria, disembark from a plane as they arrive at the Belbek military airport near the Black Sea port of Sevastopol, Crimea, Dec. 28, 2015. Reuters

Russia demanded Ukraine return two Russian soldiers reportedly abducted from the Crimean peninsula after Kiev said it picked up the deserters because they defected to Russia from the Ukrainian military following Moscow's annexation of the Crimean peninsula. Russia called the arrest "another act of gross provocation," Reuters reported Tuesday.

The Russian defense ministry said the soldiers were "illegally detained" Sunday in the former Ukrainian region annexed by Moscow in 2014. The soldiers, Ensign Maxim Evgenyevich Odintsov and Junior Sergeant Alexander Vyacheslavovich Baranov, were transported to Ukraine's Mykolaiv region, north-west of the peninsula, BBC News reported.

Russia claimed Ukraine would use "psychological and physical pressure" to get confessions from the soldiers "for crimes allegedly committed against Ukraine." Sergei Lavrov, Russia’s foreign minister, accused Ukraine Tuesday of an “illegal provocation,” while Kremlin spokesman Dmitry Peskov said Russia was “doing everything that should be done” to secure the return of the soldiers.

Ukraine denied that it kidnapped the soldiers. Kiev said the men were detained in Ukraine.

"We did not abduct anyone - we arrested two deserters who had crossed the border at the Chongar checkpoint," said Elena Gitlyanskaya, spokeswoman for the Security Service of Ukraine, to Ukraine's Unian news agency. The men were under criminal investigation, she added.

Ukraine and Russia have maintained tense relations since Russia's annexation of the Crimean Peninsula more than two years ago. Ukraine and Western leaders have accused Russia of backing pro-Russian insurgents in eastern Ukraine.

There were more than 20,000 Ukrainian soldiers in Crimea at the time of the annexation. Only 6,010 Ukrainian military staff have since returned to mainland Ukraine.

The International Criminal Court ruled last week that Russia and Ukraine were in a de facto state of war over the annexation.

"This international armed conflict began at the latest on 26 February [2014] when the Russian Federation deployed members of its armed forces to gain control over parts of the Ukrainian territory without the consent of the Ukrainian Government," the report states.