China sent two patrol ships to islands - known as Senkaku in Japan and Diaoyu in China - on Tuesday.
This came after Japan sealed a deal to buy three of the islands from their private Japanese owner.
Washington will not take sides in the matter, said Assistant Secretary of State Kurt Campbell.
The region, he said, was the ''cockpit of the global economy'' and it was ''of utmost importance'' that peace and stability be maintained.
''The stakes could not be bigger and the desire is to have all leaders to keep that squarely in mind,'' he said, in answer to questions at a debate at the Centre for Strategic and International Studies.
Mr Campbell is America's top diplomat on East Asia.
China's defence ministry has issued a strongly-worded statement against Japan's move to buy the islands, Chinese state media reported.
Q&A: China-Japan islands row
"The Chinese government and armed forces stand firm and are unshakeable in its determination and will safeguard sovereignty over the nation's territories," ministry spokesman Geng Yansheng was quoted as saying by Xinhua news agency.
"We are watching closely the evolution of the situation and reserve the right to take reciprocal measures."
A group of about 15 protesters gathered at the Japanese embassy in Hong Kong on Wednesday, shouting slogans and burning the Japanese flags.
Tension has been brewing between the two countries for several months over the islands.
Japan controls the uninhabited but resource-rich islands, which are also claimed by Taiwan. They lie south of Okinawa and north of Taiwan, sit in key shipping lanes and are thought to lie close to gas deposits.
Japan says it is buying the islands to promote their stable and peaceful management.
"We have absolutely no desire for any repercussions as far as Japan-China relations are concerned. It is important that we avoid misunderstanding and unforeseen problems," said Chief Cabinet Secretary Osamu Fujimura.
Mr Fujimura told reporters that the government had set aside 2.05bn yen ($26m, Â£16.4m) to pay for the three islands.
The move followed a bid by the outspoken and right-wing Tokyo Governor Shintaro Ishihara to buy them using public donations - an action analysts believe would have further raised tensions with China.
The US has called for ''cooler heads to prevail'' as tension intensifies between China and Japan over disputed islands in the East China Sea.