The US's top military officer said in Japan Thursday that his troops were ready to act if North Korea turned its increasingly bellicose rhetoric into action.

"We are seeking to deter North Korea from provocation," General Martin Dempsey, chairman of the Joint Chiefs of Staff, told soldiers at the Yokota Air Base, about an hour's drive west of the Japanese capital.

"I can assure you that we are postured with our Japanese allies in order to protect our citizens, their citizens, our facilities, their facilities."

Dempsey's remarks follow a visit to South Korea and China, North Korea's chief ally, as regional tensions rise over fears about Pyonyang's recent threats to attack its southern neighbour and wage nuclear war on the United States.

The Korean peninsula was already engulfed in a cycle of escalating tensions -- triggered by the North's nuclear test in February -- when Pyongyang decided on April 3 to block all South Korean access to the Kaesong industrial zone, a rare joint project between the two countries.

Dempsey, who has also met with Chinese military officials and President Xi Jinping, said the US wanted closer ties with Beijing.

But he reiterated earlier official remarks that the US would defend Japan if it came under attack -- as Beijing and Tokyo remain embroiled in an increasingly bitter territorial dispute.

"The theme (of talks with Chinese officials) was that we both aspired to a new, different and better relationship," Dempsey said.

"But that relationship has to be approached in the context of our other responsibilities, and alliances in the region.

"Will we trade off our relationship with Japan in order to have stronger relationship with China? The answer is no. That's not going to change."

Japanese Prime Minister Shinzo Abe vowed Tuesday to "expel by force" any Chinese landing on a chain of islands in the East China Sea, which are administered by Japan as the Senkakus but also claimed by Beijing 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.

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