Japan's foreign minister said that Tokyo will immediately seek a meeting of the United Nations Security Council if North Korea fires a long-range missile, a media report said Sunday. Foreign Minister Taro Aso, speaking on a Japanese TV program, also said it would be "inevitable" for the Security Council to consider imposing sanctions on Pyongyang if it goes ahead with the missile launch, Kyodo News agency reported. Japanese and South Korean news reports said the North is escalating preparations for a missile launch that could come as early as Sunday, including loading booster rockets onto a launch pad and moving tens of fuel tanks nearby. The US and Japan have confirmed the assembly of what is believed to be a Taepodong-2 missile has been completed with two stages at the launch site, based on photos from satellites, a Japanese daily reported. The Taepodong-2 missile is believed to be the North's most advanced model with the capability to reach the United States with a light payload. Japan is not a Security Council member, while the US is among the five permanent members. On Saturday, Aso met US Ambassador to Japan Thomas Schieffer and both issued statements telling North Korea that a launch would be widely criticized and would not be in the impoverished nation's interests. Tokyo was badly rattled in 1998 when North Korea fired a missile over northern Japan into the Pacific Ocean, and the move spurred Tokyo to work with Washington - which has 50,000 troops based in Japan - on a joint missile defense system. North Korea is believed to have begun filling a ballistic missile with fuel, Japan's largest daily Yomiuri reported Sunday, citing unnamed US government officials who conveyed information Saturday to the Japanese government through unofficial channels. The move comes amid an extended impasse at international talks on the North's nuclear weapons program, which haven't met since November. The North has claimed it has a nuclear weapon, but it isn't believed to have a design that would be small and light enough to place on top of a missile. One diplomat in Washington told Yonhap that the US will employ "all measures" in case of a test fire, but indicated it will closely monitor the missile's track before taking any immediate action. The communist nation has been under a self-imposed moratorium on long-range missile tests since 1999. However, it has since test-fired short-range missiles many times, including two in March.