The Russian Defense Ministry confirmed on Friday that some Somali pirates detained one day earlier for hijacking a Russian-owned oil tanker have been released, local media reported.

Defense Ministry spokesman Alexei Kuznetsov said the release was "due to imperfections in international law."

Anatoly Kolodkin, President of the International Maritime Law Association and a judge of the UN International Tribunal on the Laws of the Sea, said the decision "was negotiated with the Russian authorities."

Kolodkin also confirmed that the captain of the warship reserved the rights to decide the future of the pirates, whether to set them free or hand them over to a foreign state, the Interfax news agency reported.

Previous reports said these captured pirates might be sent to Moscow for further proceedings.

Russian anti-submarine vessel Marshal Shaposhnikov released the Russian tanker, the Moscow University, with all 23 Russian crew members safe and sound on Thursday off the coast of Yemen.

Ten pirates were detained and one was killed, said Vladimir Markin, spokesman for the Investigative Committee of the Russian Prosecutor General's Office on Thursday.

The tanker, which was carrying 86,000 tons of crude oil to China, was captured off the Somali coast Wednesday morning.

The Piracy Reporting Center of the International Maritime Bureau said more than 210 attacks were reported last year, including 47 hijackings.