As the US, Japan, Australia, and India are getting together to cooperate on one issue or another, if there are other countries that want to participate in those discussions or those activities, the door is always open, the official said

Quad -- comprising Japan, India, Australia and the United States --- is not an alliance but a grouping of countries which are interested in strengthening a rules-based order in the Indo-Pacific region, the US has said.

The four countries had in 2017 given shape to the long-pending proposal of setting up the "Quad" or the Quadrilateral coalition to counter China's aggressive behaviour in the Indo-Pacific region.

"There's nothing about the Quad that it is an alliance. It is not formalized. There's no reciprocal obligation among the countries who are involved. It's not an organization that solicits membership," a senior administration official said during a conference call with reporters ahead of the next week's 2+2 Ministerial Dialogue between India and the US.

US Secretary of State, Mike Pompeo, and Defence Secretary Mark Esper are travelling to India to hold talks with External Affairs Minister S Jaishankar and Defence Minister Rajnath Singh. The visiting American officials would also be meeting Prime Minister Narendra Modi.

To a query, the official said the Quad ministerial meeting that they had in Tokyo in October just demonstrated the strong ties among the Indo-Pacific democracies.

"I would say it's a grouping of countries who are interested in strengthening a rules-based order in which all nations are sovereign, strong, and prosperous. It's an opportunity for us to carry out continued dialogue and to work towards mutually agreed-upon outcomes.

"It's a grouping of countries that's driven by shared interests and values rather than binding obligations. That's an example of effective multilateralism at its best," the official said, requesting anonymity.

As the US, Japan, Australia, and India are getting together to cooperate on one issue or another, if there are other countries that want to participate in those discussions or those activities, the door is always open, the official said.

The cooperation of the US and India on the Quad along with Australia and Japan really reflect a convergence of common values and interests in the region, the official said.

"We're the two largest democracies in the Indo-Pacific region, and we have shared values and interests with all four countries. It's not an exclusive grouping by any means. All four countries are interested and eager in welcoming other countries to the table for individual elements of cooperation that we do, some of them under the Quad framework but not all of them. That's for sure," the official said.

"It is something that's flexible and reflects our common values and interests, and I think is really for the long term," the official said.