The erroneous statement released to local authorities and media outlets at 12:16 p.m. Tokyo time (03:16 GMT) was corrected a few minutes later, with the mistake blamed on an unknown signal picked up by a military radar.
North Korea has announced it will launch what it says is a communications satellite on April 4-8 from its Musudan-ri launch site, and on Saturday official media said that all preparations for the launch had been completed.
South Korean television reports dismissed the announcement, and the weather on Saturday was not thought to be favorable for a launch.
The United States, Japan and South Korea, who have condemned the plans, believe that the secretive state is planning to test its Taepodong-2 long-range missile.
In a joint statement issued after their meeting in London on Wednesday, Russian President Dmitry Medvedev and U.S. President Barack Obama urged North Korea "to exercise restraint and observe relevant UN Security Council resolutions," which bar the North from launching a long-range missile.
North Korea says its Kwangmyongsong-2 experimental satellite will be put into orbit for "peaceful purposes."
The United States, Britain, France and Japan already have documents prepared for a Security Council session in the event of the launch going ahead.
The Japanese military is on high alert, with orders to shoot down the rocket if it is thought to pose a threat to the country.
According to the data supplied by the North when announcing the launch plans, the first stage of rocket should fall in the Sea of Japan in 130 kilometers from the Japanese coast, with the second stage flying over Japan and falling into the Pacific Ocean.
Japan's Kyodo news agency has reported that two Aegis guided-missile destroyers had set sail from Nagasaki en route to the Sea of Japan on Saturday to intercept the rocket if necessary.
The North's official news agency warned that if Japan intercepts the rocket, it will "consider this the start of Japan's war of re-invasion more than six decades after the Second World War, and mercilessly destroy all its interceptor means and citadels with the most powerful military means."
Russia on Friday again urged Pyongyang to show restraint and to avoid "fanning the flames" and escalating tensions in the region.
"Restraint in rocket activities would help allay the international community's concerns, and build trust in the context of the six-party talks on the Korean nuclear problem," official Foreign Ministry spokesman Andrei Nesterenko said.
Russia is part of stuttering negotiations on the denuclearization of the Korean Peninsula along with North and South Korea, China, Japan and the United States.
