The United States, Australia and the United Kingdom on Wednesday announced the signing of an historic defense pact, including support for Australia in developing nuclear submarines, clearly with the aim of challenging China in the Indo-Pacific.

The three countries agreed to strengthen their mutual cooperation on advanced defense technologies, including artificial intelligence, submarine systems and long-distance monitoring.

Calling it an "historic" accord, a senior US official requesting anonymity told reporters in a conference call that "I do want to ... underscore very clearly this partnership is not aimed or about any one country. It's about advancing our strategic interests, upholding the international rules-based order and promoting peace and stability in the Indo-Pacific."

President Joe Biden formally made the announcement in a statement to reporters at the White House flanked by two televisions on which Australian Prime Minister Scott Morrison and British Prime Minister Boris Johnson also announced the pact live.

The US leader emphasized that the submarines that Australia will acquire will not be equipped with "nuclear weapons," but rather will be "conventionally armed" and yet powered by nuclear reactors.

Biden called such technology "proven" and "safe."