The top U.S. military officer, Admiral Mike Mullen, says the United States remains worried that a miscalculation could "greatly heighten the stakes" in the dispute over the South China Sea.
Mullen, the chairman of the Joint Chiefs of Staff, said Friday in Tokyo that his visit to Beijing earlier this week was productive, but indicated the U.S. wants to continue to address its concerns about China's defense build-up.
China claims almost all of the South China Sea. Five other governments: Brunei, Malaysia, the Philippines, Taiwan and Vietnam, also claim all or part of the region, which is thought to contain large reserves of oil and gas.
In recent months, the Philippines and Vietnam have complained about repeated Chinese incursions into territory they claim, some of which they say involved Chinese military vessels.
During his visit to China, Mullen visited Chinese military facilities, getting a rare look at China's submarines and nuclear missiles. In May, Chinese armed forces commander Chen Bingde visited the United States and toured some U.S. military bases.
Mullen's visit to Japan is expected to include a visit to the region hit by a massive earthquake last March. Thousands of U.S. troops helped in rescue and aid efforts after the disaster. which left about 20,000 people dead or missing. Japanese officials thanked him for the assistance.
During Mullen's visit to Beijing, Chinese officials criticized the U.S. for its military drills this week with Vietnam in the South China Sea and made clear they are unhappy with Washington's position that peace in the region is part of its national interest.
Mullen also visited South Korea this week, where he said that North Korea remains a "very real" threat to peace in northeast Asia.
Also Friday, Vietnam welcomed three U.S. warships to its waters for scheduled port calls and joint training exercises.
A border guard uses his mobile phone as he stands in front of a U.S. naval ship, docked at Tien Sa port, in Vietnam's central Danang city, July 15, 2011. (Reuters)
The U.S. embassy said in a statement that the two sides will conduct seven days of training involving navigation and damage control along with dive and salvage training. No combat drills will be held.
Officials have said the port call was scheduled in advance, but it comes during heightened territorial tensions between China and Vietnam over resource-rich areas in the South China Sea.
China's military sharply criticized the U.S. on Monday for going ahead with the exercises.
Some information for this report was provided by AP, AFP and Reuters.