President Putin said the move represented NATO’s “direct involvement” in the war in Ukraine.
“I don’t think much about Vladimir Putin,” Biden told reporters ahead of a meeting with Sir Keir at the White House.
So far, the US and UK have not allowed Ukraine to use long-range missiles against targets inside Russia, fearing that this would escalate the situation.
But Ukrainian President Volodymyr Zelensky has repeatedly urged Kiev’s Western allies to allow such use, saying it is the only way to end the war.
Since Russia launched a full-scale invasion of Ukraine in February 2022, Ukrainian cities and frontlines have been bombed daily by Russia.
Most of the missiles and glide bombs that struck Ukrainian military bases, apartment complexes, energy facilities and hospitals were launched from Russian aircraft deep inside Russia.
Kiwi said not allowing these attacks to hit the bases would hamper their ability to defend themselves.
Britain has previously said Ukraine has a “clear right” to use British-provided weapons for “self-defence” following last month’s surprise border incursion in Kiev, which “does not preclude operations inside Russia”.
However, this does not include the use of long-range Storm Shadow missiles outside Ukraine’s internationally recognized borders.
The United States provided Ukraine with long-range missiles earlier this year, but like other Western allies in Kiev, they have not been approved for use against targets deep inside Russia.
Asked whether he felt intimidated by Putin’s threat to start a war with Nato, Sir Keir said the “quickest way to solve the Ukraine war” was “what Putin is actually doing”.
Keir said the meeting between the White House and Biden was an opportunity to discuss strategy on Ukraine, “not just specific steps or tactics”.
Sir Keir also added that the two discussed the situation in the Middle East, where the Israeli-Gaza war has raged for nearly a year, and “other parts of the world”.
He told reporters there would be another opportunity to discuss the issue at the U.N. General Assembly next week.
John Kirby, a spokesman for the National Security Council, said in a separate briefing Friday ahead of the two leaders’ meeting that Washington had no plans to change any of its restrictions on Ukraine’s use of American weapons to strike Russian territory.
On Friday, Moscow expelled six British diplomats, revoking their credentials and accusing them of spying.
The country’s security service, the FSB, said in a statement that it had received documents indicating Britain was involved in inflicting a “strategic defeat” on Russia. The British Foreign Office dismissed the accusations as “completely unfounded.”
British defence analyst Justin Crump told the BBC that Putin was testing both the new Labour government and the outgoing Biden administration.
“Ultimately, Russia is already supplying weapons to Britain’s enemies and is already engaging in ‘active actions’ such as subversion, espionage, sabotage, and information/cyber operations against the interests of NATO members.
“All of this could accelerate, but given the difficulties it is facing in Ukraine, Russia cannot afford to take on the entire NATO,” Mr Crump added.
