Russia's Federal Security Service arrested a man in Sevastopol, Crimea, on Thursday on suspicion of treason for allegedly handing over secret information to Ukraine about Russia's Black Sea Fleet, the RIA news agency reported.

The man, who was not identified by name, was arrested for allegedly gathering and handing over secrets to Ukrainian military intelligence about the Russian fleet in the Black Sea, the security services were cited as saying.

He could face a 12 to 20 years prison sentence if convicted.

The arrest comes amid acute tensions with Ukraine as Moscow builds up military forces near their shared border.

According to Interfax news agency, Russia's defense ministry announced the start of large-scale military drills in Crimea and the Black Sea on Thursday that it said would involve more than 10,000 soldiers and more than 40 warships.

Later on Thursday, RIA reported that Russia ordered its top army command to begin returning troops to their permanent bases inside the country from Friday, saying it had successfully completed a "snap inspection" of forces in its south and west, near the border with Ukraine.

It was unclear if the rebasing order covered all of the forces involved in that buildup.

In response to Russia's troop withdrawal, Ukraine said it's monitoring the situation in its eastern regions.

Apart from troop buildup, further tension is also escalating in diplomacy between the two countries.

On April 17, Russia detained Alexander Sosonyuk, a consul of Ukraine's Consulate General in Saint Petersburg, saying he was caught "red-handed" during a meeting with a Russian national as he sought to receive "classified" information.

The diplomat was later declared persona non grata by Russia. Ukraine has rejected Russia's accusations and expelled an unnamed Russian diplomat in retaliation.

Ukraine's Foreign Ministry on Thursday confirmed that Sosonyuk has returned to Ukraine and will work in the ministry after taking a break.

(With input from agencies)