The top US military officer told China's leaders on Wednesday that Washington is committed to defending Japan, as Beijing and Tokyo engage in intensified rhetoric over a territorial row.

General Martin Dempsey, chairman of the Joint Chiefs of Staff, is visiting China just as the dispute between Beijing and Tokyo over uninhabited islands in the East China Sea is again heating up.

Japanese Prime Minister Shinzo Abe vowed Tuesday to "expel by force" any Chinese landing on the islands, which are administered by Japan as the Senkakus but also claimed by China as the Diaoyus.

His statement came after a flotilla of eight Chinese government vessels sailed into the disputed waters, the biggest number to do so in one day since Tokyo's nationalisation of some of the islands in September.

"In the case of Japan, I was careful to remind them (China's senior leaders) that the US has certain treaty obligations with Japan, that we would honour," Dempsey told reporters in Beijing.

US officials have said that while Washington takes no side in the dispute itself, the islands are under Japan's control and thus protected under the US security treaty with Tokyo.

Dempsey also expressed concern that rhetoric between China and Japan over the rival claims is increasing the risk that the situation could spin out of control and lead to clashes.

"I think the heightened risk is a function of heightened rhetoric that could produce emotional outcomes at the tactical level, that could frankly get away from the control of the central level," he said.

A group of Japanese nationalists said Tuesday it had sent nine ships to the area around the islands, which are also claimed by Taiwan.

Dempsey said at a joint press conference Monday with Fang Fenghui, the chief of the People's Liberation Army general staff, that Washington's aim was "to be a stabilising influence in the region".

China appeared to single out the US in a military white paper last week, saying "certain efforts" to enhance military deployment in Asia "are not conducive to the upholding of peace and stability in the region".

The US is engaged in a "pivot" to Asia after years of conflict in Iraq and Afghanistan.

"A fair share of the questions that were discussed in my meetings with senior leaders had to do with our rebalance to the Pacific, and seeking greater clarity on what we mean by it," Dempsey said.

"It is a strategic imperative for us over time to rebalance to the Pacific, because that's where future trends are taking us," he said.

Dempsey met new Chinese President Xi Jinping on Tuesday, with the state Xinhua news agency reporting that Xi called for the two countries as well as their militaries to foster deeper trust.