Dmitry Medvedev, deputy head of Russia's Security Council, said on Wednesday Britain was Moscow's "eternal enemy" and that any British official helping Ukraine in the war between the two countries could be considered a legitimate military target.

In response to British Foreign Secretary James Cleverly's statement that Ukraine has the right to use its force beyond its borders, Medvedev said "stupid British officials" should remember that Britain could be considered "at war" with Moscow.

"The UK is acting as an ally of Ukraine and providing it with military assistance in the form of equipment and specialists, de facto leading an undeclared war against Russia," Medvedev said on Twitter.

"In this case, any of its government officials (whether military or civilian, aiding in the war) can be considered a legitimate military target."

British Foreign Secretary James Cleverly has said Ukraine has the right to attack targets on Russian territory for self-defense.

Cleverly, who belongs to the ruling Conservative Party, said during a news conference with his Estonian counterpart Margus Tzhaga in the Estonian capital Tallinn on Tuesday: "Ukraine has the legitimate right to defend itself, it has the legitimate right to do so within its borders, of course, but it also has the right to use its force outside its borders to undermine Russia's ability to use force inside Ukraine itself."

"Therefore, legitimate military targets outside its borders are part of Ukraine's self-defense, and we must admit that."