The US Secretary of State, Hillary Clinton, has issued a veiled warning to China not to challenge Japan's control of disputed islands in the East China Sea.
Japan's new government has vowed not to aggravate the situation.
Ms Clinton met with Japan's Foreign Minister Fumio Kishida in Washington. She says Japanese Prime Minister Shinzo Abe will visit in February.
China and Japan have been engaged in a tense dispute over islands known as Senkaku in Japanese and Diaoyu in Chinese.
Amid signs that China is testing control over the virtually uninhabited islands, Ms Clinton said the area was under Japan's control and hence protected under a US security treaty with Tokyo.
"We oppose any unilateral actions that would seek to undermine Japanese administration," she said.
Clinton did not mention Beijing directly in the warning, but said: "We want to see China and Japan resolve this matter peacefully through dialogue."
"We do not want to see any action taken by anyone that could raise tensions or result in miscalculation that would undermine the peace, security and economic growth in this region," Ms Clinton said.
China has repeatedly criticised the US position.
Chinese surveillance ships and state-owned planes have increasingly neared the area, in what some see as a bid by Beijing to contest the notion that Japan holds effective control.
"The frequency and scale of their provocations have drastically increased," Japanese Foreign Ministry spokesman Masaru Sato told reporters in Washington.
"The Chinese are trying to change the existing order by coercion or intimidation," he said.
Japan's Foreign Minister has welcomed Ms Clinton's support, saying that the statement on the security treaty "will go against any unilateral action that would infringe upon the administration rights of Japan."
"While Japan will not concede and will uphold our fundamental positions that the Senkaku islands are an inherent territory of Japan, we intend to respond calmly so as not to provoke China," Fumio Kishida said.
US officials and pundits have largely welcomed the return of the Liberal Democratic Party, believing that Shinzo Abe's firm positions and pledges to boost military spending will deter confrontational moves by Beijing.
Separately, Ms Clinton said the United States and Japan wanted "strong action" at the UN Security Council on North Korea, which put a satellite into orbit last month in a launch the two allies fear could bolster Pyongyang's missile capabilities.
Diplomats at the United Nations said the United States and China, North Korea's main ally, had reached a compromise under which the Security Council would expand existing sanctions against Pyongyang.
The talks between Ms Clinton and Mr Kishida also focused on the hostage crisis in Algeria, with the two diplomats pressing the North African nation to release more information about a massive kidnapping at a desert gas field.