By David Brunnstrom

WASHINGTON (Reuters) - Adding partners to the AUKUS defense project would be "complicated," a senior Australian diplomat said on Friday, saying he did not believe the U.S. Congress is open to expanding the pact involving Australia, the U.S. and Britain.

AUKUS was formed in 2021 to counterbalance China's growing power. A first "pillar" involves cooperation between the three partners to provide Australia with nuclear-powered submarines, but they have raised the possibility of other countries joining a second pillar to develop other high-tech weaponry.

Paul Myler, deputy head of mission at Australia's embassy in Washington, said that it was an "almost inconceivable" achievement that after 40 years of effort to reform U.S. export control restrictions, the partners were on the verge of creating an environment for easier collaboration and co-development.

Myler noted that the three AUKUS partners had committed to talk to Japan about what might be possible in terms of collaboration, and Japan had shown the political will to field defense capabilities in a timely fashion.

"So ... there's a fundamental alignment there," he said. "But I think I need to be really clear: My shorthand for this is not Japan as being invited into AUKUS. AUKUS is reaching out to Japan, to find some collaboration that we can do out there."

"I don't think Congress would consider it (AUKUS) open for expansion, but AUKUS partners can certainly reach out and do collaboration out there," he said.

There has been some hesitation about involving Japan, with officials and experts highlighting its cyber and information security vulnerabilities.

At the end of April, the U.S. State Department unveiled a plan to reduce licensing requirements for transferring military gear and sensitive technology among AUKUS partners. It said on April 19 it expects to finalize trade exemptions for AUKUS in the next 120 days.