North Korea is assembling a long-range missile capable of reaching Alaska, a news report said Wednesday, as the communist regime prepared to test-fire a barrage of missiles from both coasts.

The moves further heightened tensions in the region following North Korea's underground nuclear test last week, and came as speculation grows that leader Kim Jong Il has selected his third son as heir to the secretive communist dynasty.

At the border village of Panmunjom inside the Demilitarized Zone separating the two Koreas, a military guide warned tensions are running deep.

"The possibility of armed provocation is higher than ever in the Joint Security Area," said the South Korean military guide taking journalists on a tour of the border area. He did not provide his full name saying he did not have permission to do so.

The guide also cautioned journalists not to "point at the North Koreans or make any gestures." On the North Korean side of the area, a lone guard could be seen standing with his arms to his side, just watching the journalist group.

The mass-market JoongAng Ilbo newspaper said Wednesday that a long-range missile — transported by train to the Dongchang-ni launch site near the North's northwest coast near China — is believed to have entered an assembly building.

The paper cited an unnamed South Korean official saying the missile was covered up and a little longer than the Taepodong-2 rocket the North fired on April 5.

An American military official confirmed an intercontinental ballistic missile capable of striking the U.S. was being readied at a base on the North's west coast. The official said it could be more than a week before Pyongyang was ready to launch. He spoke on condition of anonymity because it was an intelligence-related issue.

Separately, North Korea may soon launch three or four mid-range missiles, believed to be modified versions of its Rodong series, from its east coast, South Korea's Yonhap news agency reported.

The U.N. Security Council is considering measures to punish the North for the nuclear test, and U.S. Deputy Secretary of State James Steinberg said after meeting with South Korean Vice Foreign Minister Kwon Jong-rak he still has hope the North will change course.

"We have demonstrated that this process of consultation is in fact a way to send a clear message to North Korea," he said in Seoul. "It should reverse its course on its nuclear program and then return to the path of denuclearization that we have agreed on."

At the U.N., North Korea's allies China and Russia raised questions about some possible new sanctions against North Korea, diplomats said, with one of them describing the issues as a lot of little sticking points. They spoke on condition of anonymity because the consultations are private.

Pyongyang has countered it will not accept any punishment and has warned it won't respect the 1953 truce that ended the Korean War if it is provoked.

Fearing skirmishes off its coast, South Korea, whose troops are already on high alert, sent a high-speed ship equipped with guided missiles to its western waters, where the North was reportedly staging amphibious assault training.

South Korea is also sending coast guard ships to escort fishing boats near the western sea island of Yeonpyeong.

The long-range missile being prepared by the North — believed to have a range of up to 4,000 miles — could be timed to coincide with a June 16 summit in Washington between South Korean President Lee Myung-bak and President Barack Obama.

Complicating the situation is Thursday's trial in Pyongyang of two American journalists, Laura Ling and Euna Lee, accused of entering the country illegally and engaging in "hostile acts."

North Korea also has custody of a South Korean worker detained at a joint industrial complex at the border. He has been transferred to Pyongyang, Yonhap said Tuesday. It said North Korea has refused to allow the delivery of daily necessities to him.