Britain is focused on helping protect Ukrainians from the war and is not aiming to remove Russian President Vladimir Putin, Prime Minister Boris Johnson said on Wednesday (local time).

US President Joe Biden has said that Putin "cannot remain in power", later clarifying that his words reflected his moral outrage at Russia's invasion of Ukraine and not a US policy shift. 

Asked about Biden's comment, Johnson told a committee of lawmakers he understood "the frustrations that people feel about Putin".

"To desire a change of government in itself is not an ignoble thing ... that's the objective of a lot of democratic politics," Johnson said.

"But let's be absolutely clear, it's not the objective of the UK government, and it's very, very important that everybody gets this. We are simply setting out to help to protect the people of Ukraine, and to protect them against absolutely barbaric and unreasonable violence."

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Johnson said Britain was looking at "going up a gear" in its support for Ukraine, including possibly providing armoured Land Rovers to help the Ukrainians provide humanitarian relief to encircled cities such as Mariupol.

Some G7 leaders, including French President Emmanuel Macron, have maintained direct dialogue with Putin since Russian forces invaded Ukraine on Feb. 24.

Asked whether Macron had been speaking to Putin for too long, Johnson said the unity of the G7 and NATO should be prioritised, but the question of "the value of those negotiations is an open one. My view is that Putin is plainly not to be trusted."

He also said he did not think the G7 and NATO should be expected to lift sanctions on Russia if a ceasefire was agreed in Ukraine.

"In my view, we should continue to intensify sanctions with a rolling programme until every single one of his troops is out of Ukraine," he said.