China lodged a formal complaint with Japan on Monday about fishing boats plying waters off a disputed island chain in the East China Sea, even as their foreign ministers vowed to pursue warmer ties. Beijing and Tokyo have repeatedly quarrelled over disputed maritime territories, and Japan has often voiced concern about China\'s rising defence spending and increasingly assertive stance as a naval power. \"From ancient times, the Diaoyu islands and the adjacent islands have been a part of Chinese territory and China maintains indisputable sovereignty over them,\" Chinese foreign ministry spokesman Hong Lei said in a statement. \"Any actions by the Japanese side in the waters around the Diaoyu islands are illegal and invalid,\" he said, noting that Beijing had lodged \"solemn representations\" with Japan over fishing boats operating in the area. Hong however added: \"According to our understanding, the Japanese fishing boats have already departed from the relevant maritime region.\" Last year, China and Japan locked horns over the same islands -- called Diaoyu in Chinese and Senkaku in Japanese -- after Japan arrested a Chinese captain for ramming his trawler against Japanese coastguard ships in the area. Earlier Monday, Chinese Foreign Minister Yang Jiechi and his Japanese counterpart Takeaki Matsumoto emphasised the positive when they met at the Diaoyutai guest house in Beijing, where foreign officials are often received. \"I am willing to improve China\'s relations with Japan,\" Yang said in welcoming Matsumoto, who took office in March. The Japanese minister in turn highlighted his frequent contacts with Yang since assuming his post and said stability was \"important to the two nations\". A spokesman for Japan\'s foreign ministry, Hidenobu Sobashima, told reporters that Tokyo hoped the two sides would soon begin talks on a \"legally binding instrument in resource exploration\" in the East China Sea. The area around the disputed islands -- which are also claimed by Taiwan -- has rich fishing grounds as well as possible oil and gas deposits. Sobashima also said Matsumoto -- who also met Chinese Vice President Xi Jinping on Monday -- had proposed during his meetings in Beijing that some sort of bilateral \"crisis management mechanism\" be devised. China and Japan are the world\'s number two and three economies, respectively. In June, China denied claims that one of its marine research vessels had been active inside Japan\'s exclusive economic zone, after Tokyo filed a formal diplomatic protest with Beijing. Chinese media has reported that the vessel was on a mission in the Pacific to test water for radioactive contamination after the March 11 earthquake and tsunami that damaged the Fukushima nuclear plant on Japan\'s eastern coast. Also last month, Japanese media reported extensively on a flotilla of 11 Chinese warships that sailed between Japan\'s southern islands of Okinawa and Miyako. The Chinese ships conducted drills including target practice about 1,500 kilometres (930 miles) south of Okinawa, Kyodo News reported, quoting Japan\'s defence ministry. Tensions in the South China Sea have escalated in recent weeks, with Vietnam and the Philippines protesting at what they see as China\'s increasingly aggressive stance in the strategic region.