Satellite photographs showed fuel tanks at a missile launch site in North Korea, South Korea's Yonhap news agency said on Sunday quoting diplomatic sources in Washington.
The US, Japan and South Korea have warned North Korea against conducting an intercontinental ballistic missile test after officials said there were signs a launch could take place this weekend.
Yonhap quoted diplomatic sources in Seoul as saying on Saturday that North Korea could test a Taepodong-2 ballistic missile, with an estimated range of 3,500km to 4,300km, as early as Sunday or Monday.
The Sankei Shimbun, a Japanese newspaper, reported that North Korea had directed its people to raise the national flag and watch a message on state television on Sunday, and suggested this could be linked to a possible missile test.
In 1998, North Korea shocked the world when it fired a Taepodong-1 missile over Japan and into the Pacific Ocean.
Worst case scenario
On Sunday, Taro Aso, the Japanese foreign minister, told North Korea that Tokyo would regard any missile that dropped on Japan as an attack.
North Korea declared last year
that it had nuclear weapons
"The possibility is not zero of a missile dropping on Japan. That's why we are worried," he said.
"We always have to think of a worst-case scenario."
Kyodo news agency, quoting unnamed diplomatic sources in New York, said Japan and the US would seek immediate action by the UN Security Council if North Korea went ahead with a missile test.
North Korea last year declared it had nuclear weapons but then reached a broad agreement to give up its programme in exchange for aid and security guarantees.
Negotiations, with six nations including the US and Japan, broke down in November, with North Korea refusing to return to the table unless the US drops financial sanctions imposed over alleged counterfeiting and money laundering.