The communist North says it will launch a satellite for peaceful scientific research between April 12 and 16 to mark the 100th anniversary of the birth of founding leader Kim Il-Sung.
But the United States and its allies say it is a disguised missile test and that the launch would contravene United Nations sanctions aimed at curbing North Korea's missile programme.
A successful satellite launch would burnish the image of young Kim Jong-Un as he seeks to establish his credentials as a strong leader after taking over from his father and longtime ruler, Kim Jong-Il, who died last December.
Japanese Prime Minister Yoshihiko Noda has given the green light to shoot down the rocket if it threatens Japan's territory.
Patriot missiles were Saturday deployed at the defence ministry in downtown Tokyo and at two other bases in the region to protect the greater Tokyo area and its population of around 35 million.
The ministry also dispatched three Aegis destroyers carrying interceptor missiles, reportedly to the East China Sea, where it has already deployed Patriot missiles on the southern island chain of Okinawa, beneath the rocket's forecast flight path.
"We have taken the best possible measures that we can think of at this point," senior vice defence minister Shu Watanabe told national broadcaster NHK Sunday.
China, North Korea's main ally, called for restraint.
"China is concerned and worried about the latest development on the Korean peninsula," Chinese Foreign Minister Yang Jiechi said Saturday, according to the official Xinhua news agency.
Yang was Sunday holding a meeting on regional security with his Japanese and South Korean counterparts.
Pyongyang's progress on the launch is unclear, though it may have moved the first stage of a long-range rocket on to its launch pad, one report said.
The 38 North website, a US specialist site, said an April 4 photo of the launch site at Tongchang-ri in the country's northwest indicated the first stage of the Unha-3 rocket, while not visible, may be placed in the gantry.
Pyongyang has invited foreign experts and reporters to observe the launch, which it insists is part of peaceful space research. It has not given an exact date, but Kim Il-Sung's birthday was on April 15.
The secretive and impoverished state announced the rocket plans despite agreeing on February 29 to freeze its nuclear and missile programmes as part of a deal under which the United States would deliver badly needed food aid.
North Korean expert Masao Okonogi said Japan's missile deployment was necessary to calm the Japanese public.
"The step that Japan has taken was aimed at giving psychological assurance to the public that the government has gone so far to deal with North Korea," Okonogi, a professor at Kyushu University, told AFP.
He warned, however, that North Korea could carry out another nuclear test.
"As one possibility, North Korea could conduct a nuclear test in a protest against condemnation from various countries following the missile experiment," Okonogi said.
North Korea tested nuclear bombs shortly after its previous missile launches in 2006 and 2009.
Senior vice defence minister Watanabe told Japanese reporters there was no sign Pyongyang was preparing to test an atomic weapon. However a South Korean report, citing a Seoul intelligence official, contradicted that.
"Recent satellite images led us to conclude the North has been digging a new underground tunnel in the nuclear test site... besides two others where the previous tests were conducted," Yonhap news agency quoted the source as saying.
In 2009, Japan also ordered missile defence preparations before Pyongyang's last long-range rocket launch, which brought UN Security Council condemnation and tightened sanctions against the isolated state.
That rocket, which North Korea also said was aimed at putting a satellite into orbit, passed over Japanese territory without incident or any attempt to shoot it down.