Vladimir Putin is threatening to smash Ukraine's capital Kyiv with a "massive" new missile in revenge for its use of US and British weapons to strike Russia.

The 72-year-old premier's forces are readying themselves to fire hulking RS-26 intercontinental ballistic missiles into the heart of Ukraine - sending fears of further escalation spiking in the wake of yesterday's landmark strikes.

On Wednesday, the American, Italian, Spanish and Greek embassies in Kyiv shut down as the capital braced for a Russian attack.

The US diplomatic outpost has now reopened - but reports in Russian media have left locals uneasy.

Moscow-based newspaper Moskovky Komsomolets reports that the Russian military is readying up to launch RS-26 missiles from a site in the city of Astrakhan by the Caspian Sea - far deeper inside Russia than Ukraine is able to touch, even with Storm Shadow or ATACMS missiles.

But the RS-26 has not been used in combat before, Western missile experts say.

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The "massive" rockets are thought to fly at five times the speed of sound, narrowing the odds that Ukraine's US-supplied Patriot missile defence systems could shoot them down.

Kyiv has regularly been attacked by smaller, slower Iskander missiles since the war started 1,002 days ago.

The RS-26 is fired from a ground-based launcher - and reportedly weighs some 50 tonnes, and packs a warhead three times larger than the Iskander's.