The explosion would be at an old Soviet test site first used 70 years ago in the Novaya Zemlya archipelago and could be used by Vladimir Putin as a warning to the West over Ukraine

A senior Russian lawmaker on Thursday said that Ukrainian strikes on Russia with Western missiles would lead to global war with the use of nuclear weapons and that Moscow's response would be tough with more powerful weapons.

A close ally of President Vladimir Putin warned Western governments on Thursday that a nuclear war would ensue if they gave the green light for Ukraine to use long-range Western weapons to strike targets deep inside Russia.

The agreement allowing Ukrainians to use Western weapons on Russian territory remains a topic of debate among Kyiv's Western allies. Moscow is striving to prevent this and has now issued threats to use the SS-18 "Satan" missile,