Pyongyang said a day earlier that it plans to launch a rocket carrying a "working satellite" between Dec. 10 and 22, with much of the world suspecting it is in reality testing ballistic missiles capable of carrying nuclear warheads.
The planned launch will be the North's second launch attempt under current leader Kim Jong-un, following a failed launch in April. The young leader took power following his father's death last December.
"Many countries think that this time, sanctions should be fundamentally different (from before) in terms of their scope and content," a senior presidential official said. "We have also been discussing with the U.S. possible countermeasures against the North's rocket launch since last week."
The official did not elaborate on how new sanctions would be different.
Under U.N. Security Council resolutions, North Korea is banned from any ballistic missile activity, and there have been questions about the effectiveness of additional sanctions on Pyongyang, which has already been under an array of sanctions for decades.
"It is not important whether North Korea is trying to launch a missile or a satellite. The reason the international community is concerned is because this is aimed at developing a delivery means for nuclear weapons," the official said.
China, considered the only major ally of Pyongyang, is also trying to persuade the North to back out of the launch plan, the official said, adding Beijing "would not take the North's side as the rocket launch poses a big threat to its own security."
Later Sunday, China expressed concern about the North's rocket launch plan.
Foreign Ministry spokesman Qin Gang said the North "has the right to peaceful uses of outer space, but that the right should be exercised within the limitations of the U.N. Security Council resolutions," according to Xinhua news agency.
China hopes all parties concerned can act in a way that is more conducive to the peace and stability of the Korean Peninsula and can exercise calmness so as to avoid further escalation of the situation, the spokesman said, according to Xinhua.
In a meeting with a group of South Korean reporters last week, Chinese Foreign Ministry spokesman Hong Lei also expressed concern about "any provocations hurting peace and security on the Korean Peninsula."
South Korean and Chinese top nuclear envoys also agreed during their meeting in Beijing last week to strengthen efforts to stabilize the situation and keep close watch on the North.
Senior military sources in Seoul said the North has been in active preparations for a launch.
In an attempt to correct errors that caused the failure in April, when the rocket broke apart shortly after liftoff, the communist country "has secretly pushed to invite foreign experts there, and an unidentified expert recently visited Pyongyang," a senior military official in Seoul said, requesting anonymity.
In July 2011, two North Koreans were arrested in Ukraine on charges of trying to steal secret technical information about missile engines.
"North Korea has stepped up activities for the planned launch. We've learned it is working on a booster and communication check at its Dongchang-ri launch site," a Seoul government source said, adding Pyongyang has "yet to erect the booster for the missile."
The planned rocket launch is "a kind of celebratory firework," to mark the first anniversary of its new leader, another senior government official here said.
"The hurried attempt to launch the missile in the midst of winter seems to be caused by internal factors, rather than outside ones," he said. "By firing the rocket, the North aims to parade its leader's achievements and strengthen the solidarity of its people," he said.
North Korea has conveyed a "Notice To Airmen" to Japan and other nations, saying the rocket will take off between 7 a.m. and noon on a date between Dec. 10-22, with its first stage to fall in the Yellow Sea, and its second stage in waters east of the Philippines, officials said.
But the North has not yet notified the International Maritime Organization (IMO) or the International Civil Aviation Organization (ICAO) of such details of the planned liftoff, officials said.
On Sunday, Foreign Minister Kim Sung-hwan held a meeting with senior ministry officials to discuss how to cope with the situation, officials said. The ministry decided to hold meetings with the ambassadors of the United States, Japan, China and Russia on Monday, they said.
Seoul's chief nuclear envoy, Lim Sung-nam, also plans to travel to the United States on Tuesday to discuss ways to get North Korea to drop the launch plan, officials said.
"The most desirable situation would be for North Korea to call off the launch plan," a senior foreign ministry official said. "We will concentrate a lot of effort on this."
Lim spoke by phone with his U.S. counterpart, Glyn Davies, twice immediately after Saturday's announcement by the North, officials said. The government will also strengthen discussions with Russia and Japan, they said.
"Should North Korea go ahead with a launch, the U.N. Security Council will meet and discuss this issue," the senior ministry official said. "For now, we will focus our diplomatic efforts on stopping the launch plan."