Russia has likely already sacrificed air defense on NATO borders by sending prized S-400s to shore up losses in Ukraine: UK intel

• Russia is likely moving expensive air defense systems from Kaliningrad to Ukraine, per UK intel.
• Kaliningrad is one of Russia's most "strategically sensitive" Western outposts, the intel update.

Russia has likely re-deployed several of its famed S-400 missile systems from Kaliningrad to the Ukrainian frontline, the UK Ministry of Defense said on Sunday.

It's a sign that the Kremlin has indeed sacrificed air defense capabilities along NATO borders so it can replace losses in Ukraine, where at least three S-400 systems were reported to be destroyed by Kyiv.

In an intelligence update, the UK Defense Ministry cited "exceptional Russian air transport movements" through November that indicate Moscow moved the S-400s from Kaliningrad "to backfill recent losses on the Ukraine front."

The Russian enclave of Kaliningrad is tucked above the northern corner of Poland, and is geographically separated from the rest of mainland Russia. It also shares land borders with Lithuania in the east, while Denmark sits to its west across the Baltic Sea.

So moving S-400s from Kaliningrad would be significant, the Defense Ministry said.

"As its most westerly outpost and bordered on three sides by NATO member states, Russia sees Kaliningrad as one of its most strategically sensitive regions," the Defense Ministry wrote.

If Russia is willing to withdraw defense assets in Kaliningrad, it shows the "overstretch the war has caused for some of Russia's key, modern capabilities," it added.

S-400 Triumf missile systems, also known as SA-21s, are long-range surface-to-air systems designed to destroy aircraft and missiles.

They typically consist of radars, a mobile command post, and missile launch platforms. S-400s were touted by Russia as a technological rival to the US Patriot missile system, and are believed to be priced at $500 million each.

The UK Defense Ministry had on November 9 predicted that Russia would need to start shifting S-400s along its borders to make up for air defense losses in Ukraine caused by recent strikes.

It said the strikes show that Russia's Integrated Air Defense System is struggling to defend against modern weapons supplied to Ukraine.

The US has given Kyiv around 20 Army Tactical Missile Systems, while the UK and France have sent long-range Storm Shadow and SCALP-EG cruise missiles.