A court, in the southern city of Odessa, ordered the seizure of the tanker Nika Spirit on Monday, according to documents posted by Anatoliy Matios on social media.

"The court seized the said vessel," Matios said. "We did it legally."

Ukraine's SBU security service said last week it had stopped the Russian tanker previously known as the Neyma when it entered the port of Izmail in the southern Odessa region.

Investigators seized documents onboard and questioned its 10 crew members.

But while they held the tanker, the 10 crew members were released and allowed to return to Russia.

In November, Russia opened fire on three Ukrainian navy vessels as they tried to pass through the Kerch Strait from the Black Sea to the Sea of Azov.

Ukraine says the Russian tanker was used in that confrontation, the most dangerous open clash between the two countries in recent years.

Moscow also captured 24 Ukrainian sailors manning the vessels and plans to put them on trial.

Russian President Vladimir Putin and Ukraine's new leader Volodymyr Zelensky discussed a possible prisoner swap during their first phone call this month.

The two former-Soviet neighbours have been caught up in hostilities since 2014, when Moscow annexed Crimea and supported a separatist insurgency in eastern Ukraine.

Some 13,000 people have been killed in the war.