Japan, meanwhile, stayed on alert for the launch despite the North's announcement it had extended the window for the launch -- originally set from Dec. 10 to Dec. 22 -- to Dec. 29, citing technical reasons.
South Korea planned to ask China and Russia to urge the North to scrap its launch plan.
In Washington, State Department spokeswoman Victoria Nuland said the United States remains concerned despite the postponement, saying North Korea still plans to "launch a missile or a satellite or something that would be in violation of its international obligations, that fundamentally their plans are unchanged."
She said any North Korean launch using ballistic missile technology would be in direct violation of U.N. Security Council Resolutions 1718 and 1874, prohibiting that country from conducting further nuclear tests or ballistic missile launches.
"We again call on North Korea to refrain from a launch and to comply with its obligations," Nuland said.
If the North proceeds with its rocket launch, it would be its second attempt, having failed in a similar try in April.
Nuland reminded that the April attempt was strongly condemned by the U.N. Security Council and that it was made clear the council was determined to take action if there was another launch attempt.
Nuland said U.S. Secretary of State Hillary Clinton also has been in direct contact with the foreign ministers of China and Russia to exchange views and express concerns about the North's plans.
In Japan, as part of the government's alert response to the North's launch effort, Prime Minister Yoshihiko Noda and other Cabinet ministers were at their offices by 7 a.m. Tuesday, Kyodo News reported.
In South Korea, Yonhap News quoted Vice Finance Minister Shin Je-yoon as saying the ministry would be closely watching for any likely impact of the rocket launch on its financial markets. He said the government may call for an emergency meeting if market anxiety heightened.
Separately, a government official told Yonhap South Korea planned to hold separate talks with China and Russia to see if the North can be persuaded to stop its launch.
North Korea, under the six-party talks, is committed to end its nuclear program in exchange for massive aid. The talks -- involving the two Koreas, Japan, China, Russia and the United States -- however, have remained suspended after the North walked out over U.N. sanctions.
In extending its launch date until Dec. 29, North Korea cited technical problems in the first-stage control engine module of the rocket, its official news agency KCNA reported.
A spokesman for the country's space technology department, however, said North Korea is still "pushing forward" with the rocket launch and that they are at the "final stage" of preparations.
The North has said the launch is designed to place an "Earth observation satellite" into orbit, but critics including the United States suspect it is actually planning a test of its intercontinental ballistic missiles capable of carrying nuclear warheads.
Already, destroyers from the U.S. Pacific Command have been moved into place to monitor any long-range rocket launch by North Korea.
In announcing the U.S. plans last week, U.S. Navy Adm. Samuel J. Locklear III said, "We encourage the leadership in North Korea to consider what they are doing here and the implications on the overall security environment on the Korean Peninsula, as well as in Asia."
In its preparation, Japan has said the two U.S. destroyers equipped with the Aegis combat system will work with similar Aegis-equipped destroyers of Japan's Maritime Self-Defense Force to track the trajectory of any rocket launched by the North.
The Pentagon said North Korea, which claims to have conducted nuclear device tests in the past, has been pursuing nuclear technology and that launching the rocket could show it has a delivery system for a nuclear weapon.
Russia already has urged North Korea not to proceed with the rocket launch. China, North Korea's main ally, also has expressed concern about the launch, saying while the North has a right to peaceful uses of outer space, that right should be exercised within limitation of the U.N. Security Council resolutions.
North Korea is now led by the young but relatively unknown Kim Jong Un, who took over after the death of his father last December. Since then, the new leader has been consolidating his power.
Some experts told Yonhap the North Korean launch plan may be designed for domestic reasons related to strengthening the young Kim's rule and that December will mark the first anniversary of the death of Kim's father, Kim Jong Il.
China Daily, an official Chinese news service, commenting on the launch postponement, quoted Wang Junsheng, a researcher in East Asian studies at the Chinese Academy of Social Sciences, as saying Pyongyang is placing growing emphasis on external affairs.
"It must have made a necessary evaluation of the various responses from the international community after sending the message about the December satellite launch," Wang said.
"The new launch plan is viewed as a double-edged sword for the country, and Pyongyang's decision to postpone the deadline displays both technical and diplomatic prudence."
On Monday, the Chinese Foreign Ministry urged joint efforts to safeguard peace and stability on the Korean Peninsula.