A senior American diplomat warned Wednesday that North Korea's nuclear test, barrage of short-range missiles and preparations for a long-range missile launch have put the regime on the wrong path and urged Pyongyang to return to disarmament talks.

Deputy Secretary of State James Steinberg said he and a team of high-ranking officials were in Asia to send a clear message to North Korea that the U.S. and its allies stand united on the need for nuclear disarmament.

"I think we have a common view that we need to take steps to make clear to the North that the path it's on is the wrong one," he said.

North Korea must "reverse its course on its nuclear program and then return to the path of denuclearization that we have agreed on," he said at the start of talks with South Korea's Vice Foreign Minister Kwon Jong-rak.

The envoys' trip to Tokyo, Seoul and Beijing comes amid heightened tensions over North Korea's underground nuclear test last Monday and reports the regime is preparing to test-fire a long-range missile as well as an array of medium-range missiles.

Assembly is believed to have begun on an intercontinental ballistic missile — one capable of striking the U.S. — that was transported by train to the Dongchang-ni launch site near North Korea's northwest coast, South Korea's JoongAng Ilbo newspaper said, citing an unnamed government official.

The missile remains covered but appears longer than the Taepodong-2 long-range rocket North Korea fired on April 5, the official told the newspaper.

A U.S. military official has said it could be more than a week before liftoff. He spoke on condition of anonymity because it was an intelligence-related issue.

The saber-rattling is particularly unsettling because two American journalists, Laura Ling and Euna Lee, are set to go on trial Thursday in Pyongyang. The reporters — from former Vice President Al Gore's Current TV media venture — were accused of entering the country illegally and engaging in "hostile acts."

North Korea also has custody of a South Korean worker detained at the Koreas' joint industrial complex in Kaesong, just across the border. South Korea has accused the regime of refusing access to the man, who has been transferred to the capital, Pyongyang, according to the Yonhap news agency.

Steinberg, Secretary of State Hillary Clinton's No. 2, said Washington is ready to "enter an effective dialogue that will really lead to a complete and verifiable denuclearization of the peninsula" if the North is prepared to change its course.

He also met with South Korean Foreign Minister Yu Myung-hwan. Meanwhile, President Barack Obama's special envoy on North Korea, Stephen Bosworth, held talks with his South Korean counterpart, Wi Sung-lac.

Bosworth said he has "some confidence that at some point we're going to see" dialogue with the North resume, but did not elaborate on the basis for his optimism. He said the Obama administration has supported dialogue from the beginning and that the North will ultimately understand having dialogue is in their interest.

Obama discussed the nuclear crisis with Chinese President Hu Jintao in a phone call early Wednesday, the official Xinhua News Agency reported.

Analysts say the test of a long-range missile believed to have a range of up to 4,000 miles (6,500 kilometers) could be timed for the June 16 White House summit between Obama and South Korean President Lee Myung-bak.

"Since North Korea is faced with the benign neglect of the U.S., the best way to attract attention is to be hawkish," said analyst Lee Sang-hyun of the Sejong Institute think tank outside Seoul.

He said North Korea has a pattern of creating "an environment that is most advantageous strategically" — and then starts negotiating.

There are further signs of North Korean belligerence, with its military reportedly strengthening its defenses and conducting amphibious assault exercises along its western shore in possible preparations for a maritime skirmish.

South Korea, which put its troops on high alert, sent a high-speed ship equipped with guided missiles to the waters off the west coast. Seoul is also seeking to buy dozens of advanced ship-to-air missile interceptors from the U.S. to bolster its naval defenses.

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

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

The guide also cautioned reporters 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, watching closely.

Diplomats at the U.N. Security Council are continuing discussions on how to punish the North for the nuclear test.

However, North Korean allies China and Russia questioned pressing new sanctions against the isolate regime, diplomats at the U.N. 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 said it will not accept any punishment and has warned it won't respect the 1953 truce that ended the fighting during the three-year Korean War if it is provoked.