Japanese Foreign Minister Taro Kono and his Chinese counterpart Wang Yi were at odds Thursday over issues related to the East China Sea despite a recent improvement in bilateral ties.

During his 30-minute talks with Wang in Singapore, Kono also voiced concern over China's military buildup in the South China Sea, where Beijing and several Southeast Asian countries have overlapping maritime claims, the Japanese government official said.

Kono conveyed to Wang Japan's unease about China's moves in the East China Sea, such as the sailing of Chinese naval vessels in the vicinity of the disputed Senkaku Islands and resource development in the contested waters.

Wang responded to Kono by saying, "Naturally, China has its own stance" on the matter, while Kono emphasized that concrete progress in the maritime field is necessary to put Japan-China relations on a "development track," according to the official.

Japan has repeatedly called on China to stop unilateral resource exploration in the East China Sea, as their negotiations based on a 2008 bilateral accord on joint gas development around the line have been on hold.

In June, Japanese and Chinese defense authorities launched a communication mechanism to avert accidental clashes at sea and in the air.

But the two sides have sidestepped the territorial dispute over the Senkaku Islands, called Diaoyu in China, which escalated after the Japanese government effectively put them under state control in September 2012.

Bilateral relations have been clearly improving as this year marks the 40th anniversary of the signing of a friendship treaty.

When Japanese Prime Minister Shinzo Abe met Chinese Premier Li Keqiang in Tokyo in May, Abe expressed a desire to visit China by the end of this year.

Abe and Li also pledged to make efforts to resume reciprocal visits by the two nations' leaders.

On Thursday, Kono and Wang, who are visiting Singapore to attend a series of Association of Southeast Asian Nations-related gatherings, agreed to work together to realize Abe's visit to China later this year.

Sharing the view that stronger bilateral cooperation will contribute to international stability, Kono and Wang have confirmed the necessity of high-level exchanges, according to the Japanese official.

The two ministers, meanwhile, agreed that it is important to maintain the free trade system, the Japanese official said, as some countries -- particularly the United States -- have shifted toward unilateralism and protectionism.

China and the United States are engaged in a full-fledged trade war, with the world's two largest economies hiking tariffs on products imported from the other.

On Wednesday, U.S. Trade Representative Robert Lighthizer said he will contemplate more than doubling the proposed tariffs on $200 billion of Chinese goods to 25 percent as instructed by President Donald Trump.

Wang criticized Washington, telling reporters following the meeting with Kono that the United States has been violating principles of the World Trade Organization and going against the tide of the times.

The Trump administration has been "trying to deal with trade disputes among nations in accordance with domestic laws and solve the country's issues in a unilateral way," Wang said.

The United States had already slapped additional 25 percent duties on $34 billion of Chinese imports last month -- the first round of the new tariffs worth $50 billion -- in response to China's alleged intellectual property and technology theft.

Kono and Wang did not get into deep discussions on the situation surrounding North Korea due to lack of time, the official added.