SEOUL, South Korea —  North Korea is readying an advanced missile designed to reach the United States for a launch that could come within weeks, reports said Monday, ratcheting up tensions after its second underground nuclear test.

The reclusive communist country also reportedly bolstered its defenses and conducted amphibious assault exercises along its western shore, near disputed waters where deadly naval clashes with the South have occurred in the past decade.

Satellite images and other intelligence indicated the North had transported its most advanced long-range missile to the new Dongchang-ni facility near China and could be ready to be fired in the next week or so, South Korea's Yonhap news agency reported.

The missile being prepared for launch was believed to be an intercontinental ballistic missile with a range of up to 4,000 miles (6,500 kilometers), the JoongAng Ilbo newspaper reported, citing an unnamed South Korean official. That distance would put Alaska and U.S. bases on the Pacific island of Guam — along with all of Japan — within striking range.

Experts said North Korea's nuclear capabilities, while improving, still do not pose a direct threat to its neighbors. The larger concern, they say, is that the North will try to sell its technology to others.

Lee of the Sejong Institute said the international community's options were few. "If North Korea is determined to become a full nuclear state, there's nothing the international community can do about it," he said.