The defense ministers of Japan, the United States, Australia and the Philippines agreed Saturday to step up security cooperation in an effort to promote a free and open Indo-Pacific amid China's growing maritime assertiveness in the region.

The meeting, which took place on the sidelines of the annual Asia Security Summit in Singapore, was the first-ever defense ministerial talks involving all four countries.

(From L) Australian Defense Minister Richard Marles, Japanese Defense Minister Yasukazu Hamada, Philippine Defense Minister Carlito Galvez and U.S. Defense Secretary Lloyd Austin pose for a photo ahead of their talks in Singapore on June 3, 2023. (Kyodo) ==Kyodo

The ministers "affirmed that they have a vision for a 'Free and Open Indo-Pacific' and collectively make efforts to ensure the vision continues to thrive," according to the Japanese Defense Ministry.

Japanese Defense Minister Yasukazu Hamada, U.S. Defense Secretary Lloyd Austin and their Australian and Philippine counterparts Richard Marles and Carlito Galvez "discussed regional issues of common interest and opportunities to expand cooperation," it said.

Among the plans being considered are joint maritime patrols by vessels of the four countries in the South China Sea, with the apparent aims of monitoring the movements of the Chinese military and acting as a deterrent, according to a diplomatic source.

The meeting came as Tokyo, Washington and Canberra have all ramped up their defense cooperation with Manila amid mounting concerns over Beijing's growing military presence in the South and East China seas and intensifying U.S.-China tensions.

Japan and the Philippines have both been facing challenges related to China's claims over the Senkaku Islands in the East China Sea and almost all of the South China Sea, respectively.

There has long been tension between the Philippines and China over the resource-rich South China Sea. The current government under Ferdinand Marcos Jr. has been pursuing a different approach from his predecessor Rodrigo Duterte, who had sought closer ties with China and suggested severing his country's defense pact with Washington.

Earlier this year, the two Asian U.S. allies agreed to work toward signing a treaty designed to facilitate joint exercises and reciprocal visits between their forces.

Tokyo has also provided patrol vessels to Manila and exported an air radar system to the Southeast Asian nation.

Earlier Saturday, Hamada, Austin and Marles held a separate meeting where they pledged to increase the number of joint exercises conducted by their forces and expand their activities, according to their joint statement.

In a veiled criticism against Beijing's maritime assertiveness in regional waters, Tokyo, Washington and Canberra also expressed "serious concern about the increasingly severe security environment in the East China Sea."

They also referred to the situation in the South China Sea, expressing their opposition to the "militarization of disputed features, the dangerous use of coast guard vessels and maritime militia, and efforts to disrupt other states' offshore resource exploitation activities."

The three ministers said they "strongly object to China's claims and actions that are inconsistent with international law," adding they "undermine international rules, standards, and norms."