Ukraine's Foreign Ministry earlier cited the ship owner, Tomax Team Inc., as saying there were three Russian, 17 Ukrainian and one Latvian nationals on board the Faina ship, which was flying the Belize flag.
President Viktor Yushchenko has urged his government to do everything possible to ensure the crew's release.
Pirates are increasingly active in the waters off Somalia, which has no effective government and no navy to police its coastline. The International Maritime Bureau said more than 30 incidents of piracy were registered in the region in 2007. More than 30 attacks have been committed so far this year off the coast of the East African nation.
Russia's Navy has sent a missile frigate to waters off the Somali coast to fight piracy in the region, a Navy spokesman said Friday.