Russia appears to be strengthening its air defenses around Moscow at the expense of the front lines in its war against Ukraine, according to an analysis by the Institute for the Study of War think tank.

The move comes as Kyiv continues firing attack drones at targets in the capital region, prompting emergency flight restrictions and temporary shutdowns at all of Moscow’s major airports.

The Kremlin installed a Pantsir air defense system on a tower near the Moscow Refinery. It includes a protective metal case similar to those found in occupied Ukraine, prompting military experts in Kyiv to assess that it was deployed from the front line to Moscow, the Institute for the Study of War said.

“ISW continues to assess that Ukraine’s intensifying long-range and intermediate-range strike campaigns are imposing additional competing requirements for air defenses across Russia’s operational and deep [rear areas,], compounding Russia’s air defense challenges given the vast amount of land and large number of facilities Russia must protect,” the think tank reported Monday.

The think tank said online imagery of the deployed Pantsir system shows it has only two of the standard launch missiles on one side, suggesting that Moscow may be contending with a shortage of interceptor missiles.

The think tank cited a recent report from CBS News that said Russia is experiencing a shortage of S-300 air defense missiles due to Western sanctions limiting its supply of key components.

Analysts say Ukraine’s airstrike campaign has forced the Kremlin to make difficult decisions about where to allocate its limited air defense systems.

“Ukrainian strikes disabling Russian air defense radars over the past months are also likely contributing to Russia’s inability to sufficiently repel strikes even in areas with increased air defense protection,” the think tank reported.

Kyiv’s strike campaign against Russian oil refineries since March has contributed to gasoline shortages across occupied Ukraine and regions within Russia. Authorities in Moscow have struggled to address the shortages, the think tank reported.