Foreign Minister Taro Aso said Tokyo was ready to slap sanctions on the North, which surprised the world by firing a missile over Japan in 1998 without warning. He said any repeat launch would lead immediately to the UN Security Council.

"If they failed and the missile dropped on ... Japan, things would be complicated," Aso said on Japanese television. "It will be regarded as an attack."

He also said that the legal procedures were already in place to impose economic sanctions against North Korea.

"The next step is to put that in motion," Aso said.

Reports of the imminent test of a long-range missile with the range to hit parts of the United States have drawn stiff warnings from Washington as well as from Japan and South Korea.

Japan's Sankei Shimbun newspaper, citing unnamed Japanese government sources, said citizens of the Stalinist state had been advised to raise the national flag earlier at 0500 GMT and watch a message on television.

But that time passed without word of any launch, and South Korea's Yonhap news agency cited an official as saying that a similar call to citizens was issued last year on June 18 as part of an unrelated domestic anniversary.

Senior officials of Japan's Defense Agency also said North Korea was unlikely to test-fire the missile on Sunday, according to Jiji Press news agency.

"There won't be a launch today," one official was quoted as saying without elaborating.

Defense Agency chief Fukushiro Nukaga separately said the agency had not observed "any particular change" that would indicate an impending launch.

But "we are taking every possible measure to collect information for 24 hours a day," Nukaga said.

Nukaga said he ordered the defense agency to be on alert, hinting that its Aegis destroyers would remain deployed in the Sea of Japan and the Pacific for surveillance activities.

North Korea last year said it had nuclear weapons and since November has boycotted six-nation talks on its atomic drive, saying it will not come back to the bargaining table until the United States lifts sanctions on it.

Thomas Schieffer, the US ambassador to Japan, said Saturday there were signs the North was preparing a missile launch and warned that such a move would be "grave and provocative."

But Schieffer declined to give a time frame for a launch of a Taepodong-2 missile, which has a range of 3,500 to 6,000 kilometers (2,200 to 3,750 miles).

In South Korea, a defense ministry spokesman declined to comment on any North Korean preparations but said his country's military alert level had not been changed.

"The military is on the same level of alert as usual," the spokesman said.

In the six-nation talks -- which group North and South Korea, China, Japan, Russia and the United States -- the North had agreed to give up its nuclear program in exchange for aid and security guarantees.

But the talks have been stalled since Pyongyang said the United States would first have to drop financial sanctions imposed over alleged counterfeiting and money-laundering.

Analysts have speculated that North Korea -- which US President George W. Bush in 2002 branded as part of an "axis of evil" with Iran and Saddam Hussein's Iraq -- is trying to regain the limelight while much international attention is focused on Iran's nuclear program.

South Korea, which has been working to reconcile with its estranged northern neighbor, has also warned it against a missile launch.

In August 1998 North Korea tested a Taepodong-1 missile with a range of up to 2,000 kilometers.

At the time Pyongyang called it a satellite launch. But in 2002 it agreed not to test further long-range missiles in a declaration with Japan paving the way for the normalization of relations.

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