Seoul, South Korea ? North Korea appeared close Sunday to test-firing a long-range missile capable of reaching the United States, prompting the White House to warn of an appropriate response and Japan to threaten a “fierce” protest to the United Nations.

North Korea was silent on the issue but vowed to bolster its “military deterrent” in a burst of fiery rhetoric carried by its state news agency.

A test launch of what is believed to be a Taepodong-2 missile would inflame a region already tense over the North’s nuclear weapons program.

“There are signs” of an imminent missile launch, Jung Tae-ho, a spokesman at the South Korean president’s office, told The Associated Press. He added that security officials were “closely watching the situation.”

The North last conducted such a launch in August 1998. Pyongyang imposed a moratorium on testing long-range missiles in 1999.

The White House spokesman said Sunday the United States expected the North to abide by that freeze.

“We do not want to have a missile test out of North Korea,” Tony Snow told “Fox News Sunday.” “The North Koreans themselves decided in 1999 that they would place a moratorium on this kind of testing, and we expect them to maintain the moratorium.”

Snow noted that North Korea made a series of commitments in six-nation talks over its nuclear program in September, including that they would “bargain in good faith.”

“We expect them to come back to the table,” Snow said. “And we hope there’s not going to be a launch.”

President Bush, national security adviser Stephen Hadley and Secretary of State Condoleezza Rice have been in contact with more than a dozen countries and communicated with North Korea through the U.N. representative, Snow said.

“If they go ahead with a test, then we will have to respond properly and appropriately at the time,” Snow told CNN’s “Late Edition.” Asked if he could explain what that meant, Snow replied, “No.”

“Japan is reporting that, as far as they can tell, there are no plans to launch a missile today,” Snow told CBS’ “Face the Nation.”

The Taepodong-2 is the North’s most advance missile and is capable of reaching parts of the United States with a light payload.

In Tokyo, Japanese Foreign Minister Taro Aso said his country would take the issue to the United Nations.

“We will naturally file a stern protest and it will be fierce,” Aso told TV Asahi.

He also said it would be “inevitable” that the Security Council would consider imposing sanctions on North Korea if it goes ahead with a launch. Japan also could impose sanctions, he told Fuji TV.

Snow said he could not discuss specific intelligence about the suspected plans to launch a missile.

But Japan’s largest newspaper, Yomiuri, reported Sunday that U.S. and Japanese officials have confirmed that North Korea has assembled what is believed to be a Taepodong-2, with two stages at the launch site, based on photos from satellites.

Yomiuri and South Korea’s Yonhap news agency carried similar reports citing unidentified U.S. and diplomatic officials saying North Korea may have begun fueling the missile.

North Korean officials talked Sunday about increasing the country’s “military deterrent” during a meeting celebrating the anniversary of leader Kim Jong Il starting work in the communist party, the North’s official Korean Central News Agency said.

“Military deterrent” commonly refers to the nuclear program, which North Korea says it needs to defend itself against a potential U.S. attack. Washington denies any intention to invade.