Russia has released Somali pirates captured during the rescue of a Russian oil tanker earlier this week.
The defence ministry says the gang was freed because of "imperfections" in international law.
Russia initially said the 10 pirates would be taken to Moscow to face criminal charges over the hijacking.
The Moscow University vessel was seized on Wednesday in the Gulf of Aden, off Yemen, en route to China, carrying crude oil worth $US50 million.
Russian forces boarded the ship a day later, freeing the 23 Russian crew who had locked themselves in a safe room after disabling their ship. One pirate was killed.
The Gulf of Aden is one of the world's busiest shipping routes. Under the UN Convention on the Law of the Sea, sovereign nations have the right to seize and prosecute pirates.
But the BBC reports some countries are reluctant to do so because of concerns of what to do with the pirates after their prison terms are up.