Leaders of the United States, Japan, Australia and India have said they oppose all attempts to "change the status quo by force, particularly in the Indo-Pacific".
Tuesday's statement followed a summit in Tokyo of the four members of the grouping known as the Quad, which is united in concern over China's growing military and economic clout.
The joint statement — which comes amid concern about whether Beijing could try to forcibly seize self-ruled Taiwan — avoided any direct mention of China, but left little doubt about where its worries lie.
The carefully worded document also made reference to the conflict in Ukraine, but without offering any joint position on a military campaign that India has pointedly declined to condemn.
"As Russia's invasion of Ukraine is shaking the fundamental principles of the international order...(we) confirmed that unilateral attempts to change the status quo by force will never be tolerated anywhere, particularly in the Indo-Pacific region," Japan's Prime Minister Fumio Kishida said.
Kishida, US President Joe Biden, Indian Prime Minister Narendra Modi and Australian premier Anthony Albanese attended the Quad meeting in Tokyo.
The group's statement listed a range of activities that Beijing has regularly been accused of in the region.
"We strongly oppose any coercive, provocative or unilateral actions that seek to change the status quo and increase tensions in the area, such as 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 said.
The four nations are attempting to build their loose grouping into a more substantive counterweight to China's rising military and economic power, despite their differences.
They unveiled plans to invest at least $50 billion into infrastructure projects in the region over the next five years and a maritime monitoring initiative seen as intended to bolster surveillance of Chinese activities.
The moves come with worries over recent efforts by China to build ties with Pacific nations including the Solomon Islands, which sealed a security pact with Beijing last month.
The Quad met a day after Biden raised eyebrows and the regional temperature by saying Washington was ready to intervene militarily to defend Taiwan against any Chinese attack.
He insisted on Tuesday that his comments did not mean a change to Washington's longstanding "strategic ambiguity" on how it might respond to a Chinese invasion, but Beijing reacted angrily nonetheless.
And if Biden was keen to avoid being seen as changing policy, he left little doubt about where the Quad's focus lies. "This is about democracies versus autocracies, and we have to make sure we deliver," he said.
The leaders are set to meet again in person next year, in Australia.