Britain has committed £25 million ($34 million) to strengthen security in the Indo-Pacific region as part of a pact with Australia, as leaders of both countries expressed "grave concerns" about China's policies in its far western region of Xinjiang.

In a video call on Thursday, British Prime Minister Boris Johnson and his Australian counterpart Scott Morrison also called for peace and stability across the Taiwan Strait and warned Russia against invading Ukraine.

"They agreed the need for de-escalation and underscored that any further Russian incursion in Ukraine would be a massive strategic mistake and have a stark humanitarian cost," the leaders said in a joint statement after their meeting.