Japan's Prime Minister Naoto Kan has expressed concern over China's maritime activities and military build-up, amid a lingering diplomatic row.
He called on China to act as a "responsible member of the international community".
Mr Kan made the comments in his first major policy speech since surviving a leadership challenge last month.
Relations between the two countries hit a low point last month over a maritime incident near a disputed island chain.
"The rise of China has been remarkable in recent years," Mr Kan told Japan's parliament.
"But we are concerned about its strengthening defence capabilities without transparency and accelerating maritime activities spanning from the Indian Ocean to the East China Sea."
Earlier this year, China announced its military spending would rise by 7.5% in 2010, ending a long run of double digit growth.
Many experts believe the actual amount spent by China on its armed forces is far higher than the published amount.
Chinese officials say that as a proportion of GDP, China still spends less than other countries, such as the US.
The diplomatic row was touched off by the collision of a Chinese fishing vessel and two Japanese patrol boats in waters off a chain of small islands claimed by both countries.
China reacted angrily when Japan detained the captain of the fishing vessel for more than two weeks, demanding an apology from Tokyo and compensation over the incident.
Beijing suspended high-level talks with Tokyo, exports to Japan of rare earth metals were temporarily halted and four Japanese men were detained for allegedly entering a restricted military area.
Three of them have been released and returned to Japan on Friday, but a fourth remained in detention.
In another development, China's national tourism agency warned Chinese tourists to watch their safety in Japan.
The warning was issued after a bus carrying Chinese visitors to the country was surrounded by dozens of vehicles. People then reportedly started kicking the bus and shouting abuse at the tourists inside.
'Uncertainty and instability'
The seas around the uninhabited chain of islands, known as Senkaku in Japan and Diaoyu in China, may have rich oil and gas deposits.
Mr Kan repeated Japan's claim to the islands.
"The Senkaku islands are an integral part of our country, historically and under international law," he said.
He said good relations with China - Japan's largest trading partner - were vital to both countries, but said China must act as a responsible member of the international community.
Japan needed to adopt more active foreign and defence policies to deal with "uncertainty and instability that exist in areas surrounding our country", Mr Kan said.
His speech followed remarks from China's foreign ministry spokesman on Thursday urging Japan to "stop making irresponsible remarks and safeguard the larger interests of bilateral relations with concrete actions".
The spokesman, Jiang Yu, said: "We are willing to resolve our disputes through friendly negotiations but the Chinese government's and people's will and resolve are unswerving on issues involving China's territorial integrity and sovereignty."