Cheng Yonghua, China's ambassador to Japan, rejected a protest by Tokyo over Chinese vessels patrolling waters off the Diaoyu Islands.

Japanese Deputy Foreign Minister Saiki Akitaka summoned Cheng in Tokyo on Tuesday to lodge a protest against patrols conducted by four Chinese maritime surveillance ships.

The four vessels entered waters off the islands before noon on Monday and remained there for more than 13 hours, the Japanese coast guard said.

Relations between the two countries have sunk to their lowest level in years since the Japanese government illegally "purchased" part of the Diaoyu Islands in September. The islands belong to China and have been Chinese sovereign territory for centuries.

Li Xiushi, a researcher on Japanese studies at the Shanghai Institutes for International Studies, said the lack of trust between Tokyo and Beijing may further damage economic ties.

Beijing also blasted Japan on Tuesday for allowing fighter jets to violate the islands' airspace and letting its vessels enter China's territorial waters off the islands.

Foreign Ministry spokesman Hong Lei said China's patrols are regular missions conducted for administrative purposes and Beijing has lodged protests to Tokyo on a number of occasions demanding a halt to illegal activities.

Japanese Defense Minister Itsunori Onodera told his US counterpart Leon Panetta on Tuesday that countermeasures will "have to be taken'' in case of "provocations'' from China.Tokyo is eager to boost collaboration with Washington and enhance the "deterrence'' of Japan's military capability, Onodera said.

Japan may face consequences, Li warned. "If Japanese Prime Minister Shinzo Abe fails to be pragmatic, Japan may face challenges both domestically and overseas."

At a meeting of Japanese business leaders, the Chinese ambassador said that relations went through a deep freeze in 2012. Trade between the two countries from January to November fell to $302.8 billion, down 2.9 percent year-on-year.

The possibility of further escalation "still cannot be ruled out'', but the situation remains "controllable'', said Yang Bojiang, a Japanese studies specialist at the Chinese Academy of Social Sciences.

This year will witness the 35th anniversary of the Treaty of Peace and Friendship between China and Japan.

Though there is no apparent mood to celebrate the event, Cheng was guardedly optimistic over the new Japanese government.

Cheng urged the Japanese Cabinet to take a positive approach and abide by the four key political documents signed by the two countries.

"We hope that Japan meets China halfway and finds appropriate ways to ease the tension over the Diaoyu Islands through dialogue and consultation," Cheng said. "We shouldn't let our spirit sag when facing difficulties."

The Japan-China Economic Association is organizing a group of Japanese business people to visit China.

There are more than 20,000 Japanese firms operating in China, according to Yohei Kono, former speaker of Japan's House of Representatives and currently president of the Association for the Promotion of the International Trade.

"The current state of relations harms both countries. The Japanese government should improve ties with its neighboring nations, especially China," Kono said. "It is time for new wisdom to change the status quo."

Yang believed that Beijing and Tokyo will continue to seek opportunities to improve ties.