The UK is increasing its "air policing contribution" to NATO, Downing Street has said, in the wake of Russia's attack on Ukraine.
Two RAF Typhoon fighter jets and a Voyager tanker aircraft from the UK are to support continuous NATO air policing over Poland's border with Ukraine.
Two Typhoons and a Voyager from RAF Akrotiri in Cyprus are also to support air policing over the Ukraine-Romania border.
It comes as Russia has been widely condemned for the attack, during which explosions were heard before dawn in Kyiv, Kharkiv and Odesa.
World leaders have expressed deep concern at the start of an invasion that they believe could cause massive casualties, topple Ukraine's democratically elected government and threaten the post-Cold War balance on the continent.
President Vladimir Putin warned other countries that any attempt to interfere would lead to "consequences you have never seen".
Prime Minister Boris Johnson's official spokesman said: "We are increasing our air policing contribution to NATO from RAF Akrotiri and the UK to help protect our NATO allies.
"So, this is a defensive capability designed to protect the airspace of our NATO allies... they will not operate in Ukrainian or Russian airspace.
Watch: NATO activates defence plans after Russian 'act of war' in Ukraine.
"Just to emphasise, this isn't additional air support, this is the air support already committed to NATO doing additional activities."
NATO's military commanders have been granted more authority to deploy forces "wherever it is needed", in response.
Speaking at a press conference, the alliance's Secretary General Jens Stoltenberg said defence plans were now in place – with more than 100 jets on standby and 120 allied ships at sea, from the High North to the Mediterranean, demonstrating "ironclad" solidarity with alliance members – which doesn't include Ukraine.