MOSCOW (Reuters) - The Kremlin said on Sunday that amendments to Russia's nuclear doctrine had been prepared and were about to be formalised, meaning the relevant documents setting out the circumstances in which nuclear arms can be used by Moscow will be updated.

President Vladimir Putin warned the West on Wednesday that under proposed changes to the doctrine Russia could use nuclear arms if it was struck with conventional missiles and would consider any assault on it supported by a nuclear power to be a joint attack.

The changes were widely seen as an attempt by Putin to draw a "red line" for the United States and its allies by signalling that Moscow will consider responding with nuclear weapons if they allow Ukraine to strike deep inside Russia with long-range Western missiles.

Kremlin spokesman Dmitry Peskov told state TV reporter Pavel Zarubin on Sunday:

"The amendments have been prepared, and will now be formalised."

Peskov cited the international situation, escalating tensions near Russia's borders and the growing proximity of NATO infrastructure to them, and what he called the deeper involvement of Western nuclear powers' in the Ukraine war on Kyiv's side, as the backfrop for the changes to the doctrine.

(Reporting by Vladimir Soldatkin and Andrew Osborn; Editing by Andrew Osborn)