MOSCOW — A cornerstone of Russia's national air defense is temporarily down after the Defense Ministry on Monday grounded all of Moscow's 120 or so serving MiG-31 interceptors pending an investigation into the loss of a MiG-31 in the skies of Siberia earlier that day.

The Russian Defense Ministry announced Monday afternoon that a MiG-31 went down over the Siberian region of Krasnoyarsk. The plane's pilots ejected safely and the crash did not cause any damage on the ground, the Defense Ministry said, adding that the plane was not carrying weapons.

Following the announcement, a senior Russian Aerospace Forces source told the state-owned TASS news agency: "The decision has been made to suspend flights of this aircraft type until the causes and the circumstances of the crash are established."

The Defense Ministry's statement said only that "according to preliminary data," the MiG-31 went down due to an unspecified equipment failure, suggesting the plane was lost due to poor sustainment, as were seven Russian military aircraft during the summer of 2015.

The MiG-31 (NATO reporting name: Foxhound) is a high-altitude supersonic fighter built to intercept American bombers. It was introduced into service in 1981 as a derivative of the older MiG-25 interceptor. Around 120 remain in service, with reports indicating up to 80 in reserve.

The Defense Ministry signed a 30 billion ruble (US $378 million) contract with the state-owned United Aircraft Corp. on Nov. 27 for the modernization of more than 50 of Russia's remaining MiG-31s by the end of 2018.

Russia received its first batch of 24 modernized MiG-31BMs last April. According to Deputy Defense Minister Yuriy Borisov, the ministry's procurement czar, a total of 130 MiG-31s are slated for modernization in the long run.

According to Russian media reports, the modernized MiG-31 can fly to altitudes of 20,000 meters, reach a maximum speed of Mach 2.83 and has a range of 1,450 kilometers on internal fuel. Updated Zaslon fire control systems allow it to track up to 10 targets at once, and Russian media reports claim it can engage anywhere from four to six aircraft simultaneously.

The Defense Ministry appears to be banking heavily on the modernized MiG-31s to continue serving as the backbone of Russia's air defenses for the foreseeable future. Plans to relaunch production were scrapped, with the money being diverted to modernization and development of a new bomber interceptor design sometime in the future.

"It is already clear that after modernization … [the MiG-31] might be in military service for [a total] of 40 to 45 years. But that isn't even the limit," Borisov said in April. "I think we will consider the possibility of extending its lifespan to 50 years," leaving the MiG-31 in service until at least 2030.

Twitter: @mattb0401