MOSCOW (Reuters) -- A Russian submarine broke off a mission in the Black Sea on November 21 and headed back to base, apparently because of engine trouble, the state RIA-Novosti news agency reported.
The "Alrosa," a diesel-electric submarine, was on a training exercise when the problem occurred, the agency quoted a source at the Russian Black Sea fleet as saying.
The source said there was no danger to the crew of the Kilo-class vessel, the only operational submarine in Russia's Black Sea fleet based in the Ukrainian port of Sevastopol. Kilo-class submarines normally carry a crew of around 50 and operate in relatively shallow waters.
Russia's Navy could not immediately be reached for comment.
Ukraine's UNIAN news agency, quoting a Ukrainian military website, earlier reported that the "Alrosa" had sent out an SOS message on the morning of November 21 and that three military planes had been despatched to assist the vessel.
But Russia's Interfax news agency denied that report, quoting a Black Sea fleet source as saying that no SOS message had been sent.
Ukraine is opposed to Russia's Black Sea fleet staying in Sevastopol and has told the navy it will not renew its lease on the base when it expires in 2017.
Despite large budget rises in recent years, Russia's armed forces continue to suffer frequent problems from aging Soviet-era equipment and poor discipline and training.