North Korea is stepping up preparations to test fire a long-range missile, regional media reports say.
N Korea has been warned not to test fire missiles
Workers have moved booster rockets and fuel tanks to a site in the north-east, a South Korean daily reports.
Officials believe Pyongyang is planning to test the Taepodong-2, which has a range of up to 6,000km (3,700 miles) and is capable of reaching the US.
On Saturday, Tokyo reportedly issued a warning to Pyongyang via diplomatic channels not to proceed with the test.
Japan's Foreign Minister Taro Aso and US Ambassador Thomas Schieffer met on Saturday to discuss the issue.
On Friday, the US had warned North Korea such a test would be "provocative".
Officials from South Korea, the US and Japan have all suggested in recent days that a launch could be imminent.
South Korean daily Chosun Ilbo reported that booster rockets had been loaded onto a launch pad and 10 fuel tanks moved to the site in preparation for a test.
The information came from US and South Korean satellite images of activity at the site, the daily said, citing an unnamed government official.
A Japanese daily, the Sankei Shimbun, reported that the missile could be fired as early as Sunday, citing unnamed government officials.
On Friday, state department spokesman Sean McCormack said the US was monitoring the situation.
"We, of course, will take necessary preparatory steps to track any potential activities and to protect ourselves," he said.
Mr McCormack said a missile test would violate a commitment made by North Korea in 1999 to avoid such activities and would place North Korea further down a path of isolation.
North Korea last tested a long-range missile in 1998, when it fired the Taepodong-1 missile, with a range of 2,000 km, over Japan. The missile landed in the Pacific Ocean.
Diplomats say that North Korean technicians are going through the same procedures that were undertaken for the test in 1998.
Officials in South Korea initially said the preparations could be a performance to step up diplomatic pressure on Washington.
But the government has since given two public warnings to the north not to go ahead with any test.
It says a test launch would damage regional security and stall diplomatic efforts to resolve the confrontation over North Korea's nuclear weapons programme.