SEOUL, South Korea — North Korea appears to be preparing to test an advanced missile designed to reach the United States, a U.S. official said Monday, ratcheting up tensions after an underground nuclear test last month.

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.

A U.S. official confirmed the Yonhap report and said the missile was moved by train, although he did not comment on where it was moved to, and said it could be more than a week before Pyongyang was ready to launch. He spoke on condition of anonymity because the issue involved intelligence.

The activity at the launch site came as the United Nations Security Council mulled punitive action for North Korea’s May 25 nuclear test.

The missile being prepared for launch was believed to be an intercontinental ballistic missile with a range of up to 4,000 miles, 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.

Separately, South Korean media reported that North Korean leader Kim Jong Il’s youngest son, Jong Un, has been picked to be the reclusive nation’s next leader. Little is known about the 26-year-old man. He studied at the International School of Berne in Switzerland until 1998, learning to speak English, German and French, a Swiss weekly news magazine reported in March.