Russia has added a nuclear-powered submarine to its navy forces, bringing the total number of Borei-class nuclear ballistic missile submarines in its fleet to four, the Russian news agency Tass reported Tuesday. The Defense Ministry is expected to build another 11 nuclear submarines by 2020 despite concerns about the navy’s questionable underwater safety record.

The Borei-class submarine was part of a generation of Russian-made subs designed to reduce building and maintenance costs. "The submarine has successfully passed all the trials with test firing of all types of weapons,” a Defense Ministry source told Tass. “The vessel, armed with 16 Bulava intercontinental ballistic missiles, has been commissioned with the navy combat forces.” The submarine measures 558 feet (170 meters) long and can reach depths of up to 1,476 feet (450 meters).  

Russia is attempting to phase out its aging and outdated submarine fleet following a series of mishaps in recent years. Earlier this month, a 23-year-old nuclear submarine caught fire, raising fears about a possible nuclear disaster. It was the Russian navy’s seventh major accident since 2000 when the Russian submarine Kursk exploded and sank, killing everyone aboard.  

Since then, Russia has scaled back its submarine fleet from around 400 vessels to fewer than 70 in 2007. Many of the submarines that were removed from operation were more than 30 years old.

The country’s recent navy growth follows increased Russian aggression in Ukraine, circumstances that have put many countries in Eastern Europe and Scandinavia on high alert. In March, Sweden announced it would expand its submarine fleet after its navy failed to locate what was suspected to be a Russian minisub in the waters near Stockholm. Officials announced Monday that what they thought was an unknown foreign vessel was actually a workboat.

Fighting continued this week in Ukraine between pro-Russian rebels and Ukrainian government troops despite leaders’ push for a truce, the BBC reported. Officials from France, Germany, Russia and Ukraine called for more weapons to be withdrawn from the frontline.