A submarine broke off a mission in the Black Sea on Saturday and headed back to base, apparently because of engine trouble, RIA-Novosti 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 the Black Sea fleet based in the Ukrainian port of Sevastopol. Kilo-class submarines normally carry a crew of about 50 and operate in relatively shallow waters.

The Navy could not immediately be reached for comment.

Ukraine's UNIAN news agency, quoting a Ukrainian military web site, earlier reported that the Alrosa had sent out an SOS message on Saturday morning and that three military planes had been dispatched to assist the vessel.

But Interfax denied that report, quoting a Black Sea fleet source as saying that no SOS message had been sent.

Ukraine is opposed to the 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, the armed forces continue to suffer frequent problems from aging Soviet-era equipment and poor discipline and training.