Japanese and American navies are planning joint drills when United States warships arrive in the Korean peninsula this month, in a show of force to deter Pyong- yang from further provocations.
The drills will involve helicopter landings on each other's ships as well as communication exercises, two sources told Reuters.
A US naval strike force, led by the nuclear-powered aircraft carrier USS Carl Vinson, is en route to the Korean peninsula and due to arrive soon.
"Japan wants to dispatch several destroyers as the Carl Vinson enters the East China Sea," one of the sources was quoted as saying.
The upcoming foray of the US warship into waters in North Korea's backyard has led Pyongyang to declare it is ready for war.
As tensions mount, Japan's Foreign Ministry yesterday issued a travel notice for its citizens going to or residing in South Korea, urging them to take heed of developments though there was no immediate danger and no need to curtail travel.
Meanwhile, US Defence Secretary James Mattis said on Tuesday of the deployment of the warship: "There is not a specific demand signal or specific reason we are sending her up there... She operates freely up and down the Pacific and she is on her way up there because that is where we thought it was most prudent to have her at this time."
Defence expert Tosh Minohara of Kobe University told The Straits Times it was about time for even more joint drills. "The US and Japan need a lot more training at the operational level, and so this is to be welcomed in the light of the current security environment," he said.
But there are risks of increased paranoia in Beijing, which has its own military ambitions and is locked in a territorial dispute with Tokyo over a group of uninhabited islets in the East China Sea. Dr Minohara said China needs to realise that any action by the US-Japan-South Korea security triangle is "not directed at them but is a direct consequence of them not acting more forcefully towards North Korea".
The latest developments come as North Korea is said to be gearing up for a show of its prowess to mark the 105th year of founder Kim Il Sung's birth on Saturday.
Japan has found US threats of a unilateral strike to be disquieting, in the wake of the sudden attack on a Syrian airbase last week that military experts say was also aimed at intimidating Pyongyang.
Tokyo has since asked for its ally's reassurances that any military action against the North will be discussed bilaterally beforehand, the Nikkei Asian Review reported.