Sir Keir Starmer has promised Russia's "irresponsible rhetoric" on nuclear weapons will not affect UK support for Ukraine.
Ukraine has fired US-supplied longer-range missiles at Russian territory for the first time, the Russian government said, a day after the US gave its permission for such attacks.
Earlier on Tuesday, President Putin approved changes to Russia's nuclear doctrine, setting out new conditions under which the country would consider using its arsenal, treating strikes from non-nuclear states, backed by a nuclear state, like the US, as a joint attack on Russia.
Speaking at the G20 summit, Sir Keir said the UK would "ensure Ukraine has what is needed for as long as needed".