Russia has started the construction of the fourth Borey-class strategic nuclear-powered submarine designed to carry the Bulava missile, a shipyard spokesman said on Monday.

"The work on the sub construction effectively started last year," he said.

It was previously reported that construction of the Project 955 Svyatitel Nikolai (St. Nicholas) submarine at the Sevmash shipyard in the northern Russian city of Severodvinsk was delayed from December 2009 until the first quarter of 2010.

Russia's newest Borey-class strategic nuclear submarine, the Yury Dolgoruky, which is expected to be armed with the new Bulava sea-launched ballistic missiles (SLBM), is currently undergoing sea trials.

The vessel is 170 meters (580 feet) long, has a hull diameter of 13 meters (42 feet), a crew of 107, including 55 officers, a maximum depth of 450 meters (about 1,500 feet) and a submerged speed of about 29 knots. It can carry up to 16 ballistic missiles and torpedoes.

Construction costs totaled some $713 mln, including $280 mln for research and development.

Two other Borey-class nuclear submarines, the Alexander Nevsky and the Vladimir Monomakh, are in different stages of completion. Russia is planning to build eight of these subs by 2015.

Fourth-generation Borey-class nuclear-powered submarines are expected to constitute the core of Russia's modern strategic submarine fleet.

The submarine's entry into service could be delayed however by a series of setbacks in the development of the troubled Bulava missile, which has officially suffered seven failures in 12 tests.

Some analysts suggest that in reality the number of failures was considerably larger. For example, according to Russian military expert Pavel Felgenhauer, of the Bulava's 12 test launches, only one was entirely successful.

The future development of the Bulava has been questioned by some lawmakers and defense industry officials, who have suggested that all efforts should be focused on the existing Sineva SLBM.

But the Russian military has insisted that there is no alternative to the Bulava and pledged to continue testing the missile until it is ready to be put into service with the Navy.

Borey-class submarines have been exclusively designed for the Bulava, and redesigning them for the Sineva would be a major setback for the Navy's plans.