|Anti-Japan sentiment in China is running high following the continued detention of a Chinese boat captain [EPA]
Japan has said it was not officially informed of China's move to break off contacts over a maritime dispute involving the extended detention of a Chinese fishing boat captain.
A spokesman for Naoto Kan, the Japanese prime minister, said on Monday that Tokyo was assessing the situation and called on Beijing to stay calm.
Noriyuki Shikata told the Associated Press news agency that "it is truly regrettable" if China did make such a decision.
"We call for calm and prudent action by China in order not to further escalate the situation," he said.
Shikata said the investigation into the Chinese captain's case is based on Japan's domestic law and is "not based on any political intent".
The comments came a day after China's state media reported the suspension of high-level contacts with Japan in response to the boat collision incident in disputed waters on September 7.
"If Japan acts wilfully, making mistake after mistake, China will take strong countermeasures, and all the consequences will be borne by the Japanese side"
Ma Zhaoxu, China foreign ministry spokesman
There were anti-Japan protests held in several cities in China on Friday, marking the anniversary of a Japanese invasion in the country in 1931.
Al Jazeera's Melissa Chan, reporting from Beijing, said the dispute appeared to be escalating and that at some point "will hit a point of no return".
"It is not clear what that situation is yet but both countries have their citizens looking at their governments and if anyone of them backs down it will be seen as a sign of weakness," said our correspondent.
"The economy [however] is very different from the political situation, and that close relationship is not going to change.
"This is really a political fight and both governments are testing waters to see where they stand in Asia, particularly their economies."
Zhan Qixiong, the captain, and his Chinese crew were arrested after their fishing trawler hit two coast guard vessels near an island group north of Taiwan claimed by both Japan and China.
The disputed islands, known as Diaoyu in China and as Senkaku in Japan, are also claimed by Taiwan and lie in an area with rich fishing grounds. The area is also believed to have sizeable oil and gas deposits.
The 14 crew were released last week but Zhan's continued detention – for further questioning pending a decision about whether to press charges – has inflamed the prevailing anti-Japanese sentiment in China.
China announced its move to break off ties shortly after a Japanese court extended Zhan's detention by another 10 days.
The Chinese foreign ministry said Japan's refusal to release the boat captain had caused "severe damage" to relations.
A ministry statement said Beijing had suspended ministerial and provincial-level contacts, halted talks on aviation issues and postponed a meeting to discuss coal.
"If Japan acts wilfully, making mistake after mistake, China will take strong countermeasures, and all the consequences will be borne by the Japanese side," Ma Zhaoxu, a ministry spokesman, said in a separate statement.
Richard Bitzinger, a defence and security analyst in Singapore, said the two countries "have a high stake in maintaining relations".
"I don't think things have gotten that bad yet," he told Al Jazeera.
"Obviously I think the next move by Japan - what they are going to do with the captain of the Chinese ship that they are holding captive and what they do with him - is going to be a pretty important decider as to what is going to happen after that."
The latest spat is the lowest bilateral relations have fallen to since the 2001-2006 term of Junichiro Koizumi, the former Japanese prime minister, whose repeated visits to a Japanese war shrine during his tenure angered China.
The two countries halted ministerial-level defence talks for three years from 2003.
The dispute also comes ahead of a planned visit to Japan by Hu Jintao, the Chinese president, to attend the annual summit of Asia-Pacific Economic Co-operation forum leaders to be held in Yokohama in November.
Leaders of the two countries were also due to attend a G-20 summit in Seoul the same month.