US Secretary of State Antony Blinken and the foreign ministers of the other three nations assembled in New Delhi under the auspices of their Quad grouping.
In a joint statement issued by host India, the Quad called for "the importance of adherence to international law" in the East and South China Seas "to meet challenges to the maritime rules-based order".
"We strongly oppose any unilateral actions that seek to change the status quo or increase tensions in the area," it said.
"We express serious concern at the militarisation of disputed features, the dangerous use of coast guard vessels and maritime militia and efforts to disrupt other countries' offshore resource exploitation activities," it added.
China claims sovereignty over almost the entire South China Sea, through which trillions of dollars in trade passes annually.
It has ignored an international court ruling that its claims have no legal basis and built bases in the waters, alarming Washington and other Asian countries with territorial claims over its waters.
The statement did not explicitly name China, which has repeatedly accused the United States of spearheading the Quad to encircle the rising Asian power.
Its members deny hostile intentions and stress that they are not a military alliance, instead cooperating in areas such as disaster relief.
After their meeting, Japanese Foreign Minister Yoshimasa Hayashi said China had no reason to fear the Quad.
"This is not military but just practical cooperation," he said at the Raisina Dialogue, a forum in New Delhi.
"We don't try to exclude anybody. This is an open dialogue," he said.
"As long as even China abides by the laws and international norms, and also acts under the international institutions, standards and laws, then this is not a conflicting issue between China and the Quad."
The ministers said they discussed joint efforts including on building the resilience of supply chains and working together on vaccine delivery.
The latest Quad meeting, on the sidelines of a Group of 20 gathering in the Indian capital, comes weeks after Blinken scrapped a trip to Beijing following an alleged Chinese spy balloon's flight over the United States.
Washington has accused Beijing of considering supplying arms to Russia for the war in Ukraine -- allegations so far forcefully denied by China.
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