U.S. Deputy Secretary of State Stephen Biegun said Washington believes there is a need to formalize a NATO-like joint structure for security in the Indo-Pacific region.

During a press conference on Tuesday, Biegun said it is Washington's view that in the passage of time, the Quad, involving the U.S., Japan, Australia and India, should become "more regularized and at some point formalized."

Asked if the U.S. has sought South Korea's participation, Biegun said the Quad is still a somewhat undefined entity and it would be premature to talk about expanding it, adding that the U.S. has no "designed policy."

He, however, welcomed cooperation with any country in the region that is committed to defending a free and open Indo-Pacific that guarantees sovereignty and prosperity.

While the U.S. has called on countries to join the group that it earlier said would counter "aggression" from the Chinese communist party, many of its allies, including South Korea, are reluctant due to their economic ties to China.