Wedged between EU and NATO members Lithuania and Poland that have firmly backed Ukraine in the conflict with Moscow, the heavily militarised region of Kaliningrad does not share a land border with Russia.

On Thursday, "three Mig-31i aircraft with Kinzhal hypersonic missiles were relocated to the Chkalovsk airfield in the Kaliningrad region," Russia's defence ministry said in a statement.

It said that they will be on "round-the-clock combat duty".

Russian President Vladimir Putin, who unveiled the Kinzhal missile in 2018, has termed it "an ideal weapon" that flies at 10 times the speed of sound, making it extremely difficult for missile defences to intercept.

Russia in June clashed with Lithuania after Vilnius banned the rail transit of sanctioned goods from mainland Russia to Kaliningrad.

The EU, however, said Lithuanian must allow Russian goods to transit with the exception of weapons.

Seized by the Red Army from Germany in the closing stages of World War II, Kaliningrad was separated from the Russian mainland following the break-up of the Soviet Union when Lithuania became an independent state.