The Pentagon is urging Japan and Australia to clarify what role they would play if the US and China went to war over Taiwan, the Financial Times reported.

Elbridge Colby, the US under-secretary of defense for policy, has been pushing the matter during recent talks with defence officials of both countries, Saturday’s report said, citing people familiar with the discussions.

According to the newspaper, the reported request caught both Tokyo and Canberra off guard, as the US itself does not offer a blank cheque guarantee to defend Taiwan.

Reuters could not verify the report. The US department of defense did not immediately respond to a request for comment.

The US is Taiwan's most important arms supplier, despite a lack of formal diplomatic ties. Taiwan has faced increased military pressure from China, including several rounds of war games, as Beijing seeks to assert its sovereignty claims over the island. Taiwan rejects China's assertion of sovereignty.

Colby was deputy assistant secretary of defense for strategy and force development during President Donald Trump's first term. Colby is known for arguing that the US military should prioritise competition with China and shift its focus from the Middle East and Europe.