Support for Ukraine must continue until Putin withdraws: Johnson

London: Russian President Vladimir Putin must not be allowed to repeat the annexation of Crimea in other parts of Ukraine, UK's outgoing Prime Minister Boris Johnson told world leaders.

Johnson said allies should continue to support Ukraine until Russian forces withdrew from the "entirety" of its territory, reports dpa news agency.

Crimea was annexed by Russia in 2014 and Johnson warned that Putin would seek to repeat the process in other parts of Ukraine following the invasion by Moscow's forces six months ago.

In a remote address to the international Crimea Platform conference hosted by Ukraine's President Volodymyr Zelensky on Tuesday, Johnson said the peninsula had been turned into an "armed camp" by Russia since 2014 and was used as one of the launch pads for the invasion in February.

"Putin is planning to do to other parts of Ukraine, indeed all of Ukraine, what he has done to Crimea," Johnson warned.

"He is preparing more annexations and more sham referendums."

Johnson said Russia's annexation of Crimea "or any other Ukrainian territory" must never be recognised.

"In the face of Putin's assault we must continue to give our Ukrainian friends all the military, humanitarian, economic and diplomatic support that they need until Russia ends this hideous war and withdraws its forces from the entirety of Ukraine," he said.

Johnson has formed a close bond with Zelensky and is keen to reassure the Ukrainian administration that the UK's support will continue when he leaves Downing Street in September.

His address to the Crimea Platform conference came on the eve of Ukraine's independence day.

The importance of the day has led to concerns that Russia might step up military action, with the US issuing a security alert about the prospect of strikes against Ukraine's "civilian infrastructure and government facilities in the coming days".