BEIJING — Japan on Monday urged China to remain calm and not inflame their diplomatic spat further after Beijing severed high-level contacts and then called off a visit by Japanese youth over the detention of a Chinese fishing boat captain near disputed islands.China's actions pushed already-tense relations to a new low, and showed Beijing's willingness to play hardball with its Asian rival on issues of territorial integrity, which include sparring with Japan over natural gas fields in the East China Sea.Late Sunday, Beijing said it was suspending ministerial and provincial-level contacts, halting talks on aviation issues and postponing meetings to discuss energy-related issues, including a second round of talks with Japan on the gas deposits.On Monday, an official Chinese youth organization called off a visit by 1,000 young Japanese to Shanghai, piling more pressure on Japan to release the captain, who has been held for nearly two weeks after his ship collided with Japanese patrol boats in the East China Sea on Sept. 7.The tensions have sent ties to their lowest level since the 2001-2006 term of former Prime Minister Junichiro Koizumi, whose repeated visits to a war shrine in Japan enraged China. They have raised questions about cooperation between the nations at international forums such as this week's summit in New York on United Nations goals to fight poverty, which Japanese Prime Minister Naoto Kan and Chinese Premier Wen Jiabao are attending.While competitors, the economies of the China and Japan, the world's second- and third-biggest economies, have become more intertwined in recent years and there have been no signs so far the dispute would hurt business relations.Kan's spokesman, Noriyuki Shikata, told The Associated Press that China had not yet given formal notice of the suspension of contacts and exchanges."We call for calm and prudent action by China in order not to further escalate the situation," Shikata said. Any Chinese decision to suspend contacts would be "truly regrettable," he said.Shikata said the investigation into the Chinese captain's case was being conducted on the basis of Japanese domestic law without taking into account political considerations.China's Foreign Ministry, meanwhile, said Japan's actions had severely damaged relations."If Japan acts willfully, making mistake after mistake, China will take strong countermeasures, and all the consequences will be borne by the Japanese side," Chinese Foreign Ministry spokesman Ma Zhaoxu said.