The Russians have grounded their Tu-95 Bear bomber aircraft fleet after one of the aircraft caught fire while trying to take off from a base in Russia’s southwest region.

Russian officials said the country’s air force has launched an investigation to seek the cause of the fire. Officials didn’t say how long the grounding of the Bear fleet would last.

“The accident occurred during a practice flight at the Ukrainka airfield in the Amur region at 17:00 Moscow time. The Tu-95 ran over the runway during acceleration. There was no ammunition onboard. According to preliminary information, engine fire was the cause behind the accident,” the Russian ministry said in a statement.

In April, two of Russian Bear bombers flew into U.S. airspace in Alaska. This week, Russian fighters have flown over U.S. Navy ships as the Navy conducts exercises with NATO allies.