Russia added a new fleet of Su-33 fighter jets Monday to its growing military contingent in the Arctic, a Russian naval spokesman said. The Kremlin has spent the last several months bolstering its presence in the resource-rich polar region, despite repeated NATO criticism.

The unspecified number of Su-33 navy flankers will function as air defense units for the military in the Arctic, Russian agency Sputnik International News reported. The aircraft were previously used to defend the Northern Fleet ships on long excursions.

“Pilots of the shipborne fighter aviation regiment of the Northern Fleet have begun service guarding the air borders of the Kola Arctic region using the deck-based Su-33 fighter jet – the naval equivalent of the multi-role Su-27 fighter jet,” a spokesman for Russia’s Northern Fleet said in a statement. The Su-33 can operate from aircraft carriers and was designed as an air-to-air fighter.

The addition of the Su-33 fighter fleet came days after the Russian Defense Ministry announced Arctic forces would receive more than 140,000 tons of supplies by the end of 2015. Russian ships that brought the cargo to the Arctic will also carry waste out of the area on their return voyage.

Russia’s military activity has drawn increased scrutiny since President Vladimir Putin ordered the annexation of Crimea from Ukraine in March 2014. NATO has also bolstered its military presence in Eastern Europe while accusing Russia of supporting separatists in the eastern Ukraine conflict.

Putin vowed earlier this year to overhaul Russia’s military by 2020 and has spent considerable resources on upgrades to forces in the Arctic, which is rich in natural resources. The Kremlin aims to have a “self-sufficient” Arctic unit by 2018 with land, sea and air components, a defense official told Tass in April.