President Joe Biden has shifted US policy to allow Ukraine to fire missiles at targets deep inside Russia in response to North Korea's involvement in the conflict, sources say. Biden had previously resisted pleas from Ukrainian President Volodymyr Zelensky to ease limits on the use of US-supplied ATACM missiles, but Moscow's decision to deploy North Korean soldiers to Russia's Kursk region prompted a change in thinking. The relaxation of restrictions on US weapons may have come too late to alter the course of the conflict, but it could help Ukraine defend its foothold in Kursk. The move also aims to send a message to North Korea and Russia that their involvement is unacceptable.