Moscow, Sep 7 (IANS): An exchange procedure for citizens arrested and convicted in Russia and Ukraine has started, a lawyer for an inmate said on Saturday.

"Ukrainian sailors have been delivered from the Lefortovo pre-trial detention centre to Vnukovo Airport, which is why it is safe to say that the exchange has started," the lawyer of told Russia's TASS news agency.

Preparations for exchange of detainees between Moscow and Kiev intensified after Vladimir Zelensky was elected Ukrainian President.

The active stage of negotiations and formalization of legal procedures regarding the citizens of the two countries to be put on the list started a couple of weeks ago.

On September 5, Russian President Vladimir Putin said that the exchange would be large-scale and can serve as a step towards improving the bilateral relationship.

On November 25, 2018, three Ukrainian naval ships violated the rules of passage from the Black Sea to the Sea of Azov. The intruding Ukrainian vessels illegally crossed the Russian state border, trespassed into Russian territorial waters and conducted dangerous maneuvers.

In defiance of repeated warnings and demands to halt, the Ukrainian ships carried on violating the law, which drove Russian forces to fire on them in order to get them to stop. As a result, all three ships were detained.

A criminal case was opened over the violation of the Russian state border, with 24 sailors arrested.