North Korea is preparing to launch three or four medium-range missiles, along with an ICBM, amid moves by Kim Jong Il to anoint his third son as heir to the world's first communist dynasty, reports and experts said Tuesday.

South Korea's Yonhap news agency said the North is preparing to fire three to four medium-range missiles from Anbyon on its east coast.

"Many vehicles mounted with mobile launchers are being spotted" at Anbyon, Yonhap quoted an unnamed government official saying. "It looks like at least three missiles will be launched."

The North has also transported what is believed to be its most advanced intercontinental ballistic missile to a base near China in the west, officials confirmed.

Yonhap did not say when the medium-range missiles, possibly a version of the Rodong series, might be launched.

The reclusive communist country was showing other signs of belligerence. Reports say that over the past several days the North has strengthened its defenses and conducted amphibious assault exercises along its western shore that could be preparations for skirmishes at sea.

South Korea has deployed a guided-missile high-speed boat to the area to "frustrate North Korea's naval provocation intentions and destroy the enemy at the scene in case of provocations," the navy said in a statement.

The ship has guided missiles that can strike enemy vessels 87 miles (140 kilometers) away, a 76mm gun and a 40mm cannon as well as a sophisticated radar system. South Korea is also sending coast guard ships to escort fishing boats near the western sea island of Yeonpyeong.

Speculation was growing that last Monday's underground nuclear test and subsequent missile launches were related to a power shift in North Korea. On Tuesday, South Korean media and an opposition lawmaker said 67-year-old Kim Jong Il's youngest son, Jong Un, 26, has been picked to be the next leader.

The announcement to North Korea's ruling party, government and military officials came after the nuclear test, South Korean newspapers Hankook Ilbo and Dong-a Ilbo reported.

The long-range missile being prepared could be timed to coincide with a June 16 summit in Washington between South Korean President Lee Myung-bak and President Barack Obama.

The missile is believed to have a range of up to 4,000 miles (6,500 kilometers), the JoongAng Ilbo newspaper reported, citing an unnamed South Korean official. That would put Alaska and the Pacific island of Guam, which has major U.S. military assets, within range.

Satellite images indicated the North had transported the missile to the new Dongchang-ni facility near China and could be ready to be fired in the next week or so, Yonhap 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 U.N. Security Council was considering punitive action for the May 25 nuclear test. U.S. Ambassador Susan Rice said Monday that key powers were making progress on a new U.N. resolution that will almost certainly expand sanctions against North Korea for conducting a second nuclear test in defiance of the Security Council. It had conducted one in 2006.

But, complicating the situation, a trial was set to begin Thursday in Pyongyang of two American journalists, Laura Ling and Euna Lee, accused of entering the country illegally and engaging in "hostile acts."

Yonhap said Tuesday that North Korea has also moved a South Korean worker detained just north of the border in March to the nation's capital.

The Koreas ended their three-year war in 1953 with a truce, but North Korea said last week it would no longer abide by its conditions. It also disputes the U.N.-drawn western sea border, around which deadly clashes with South Korea occurred in 1999 and 2002.

No incidents have been reported in the Demilitarized Zone that separates the two Koreas, and life seemed normal on the North Korean side of the Yalu River, which marks the country's border with China.