North Korea is almost certain to launch its long-range rocket, possibly as early as Saturday if weather allows, Reuters reported South Korean President Lee Myung-bak as saying on Friday, promising a "strong and stern response."
"I think it's almost certain North Korea will fire the missile," he told reporters in London where he had attended the G20 summit.
President Obama gave North Korea a firm warning Friday, saying the provocative threat has put "enormous strains" on the international community.
"Should North Korea decide to take this action, we will work with all interested partners in the international community to take appropriate steps to let North Korea know that they cannot threaten the safety and stability of other countries with impunity, Reuters quotes Obama from a joint news conference with French President Nicolas Sarkozy.
Saturday is the start of a five-day window during which the North says it will send a communications satellite into orbit, and officials have said they think the North won't wait. The U.S., South Korea and Japan think the communist country is really testing long-range missile technology — a move they have warned would violate a U.N. Security Council resolution banning the North from ballistic activity.
The launch has sparked international alarm because the North has admitted it has nuclear weapons and has repeatedly broken promises to shelve its nuclear program or halt rocket tests.
North Korea's planned missile launch would be a "provocative act in violation" of a United Nations Security Council resolution, the White House said on Friday.
White House spokesman Robert Gibbs said "preparations" were being made in the event of the launch going ahead but declined to give any more details in comments to reporters during a trip to Europe by U.S. President Barack Obama.
U.S. officials have said they plan to take the issue to the Security Council if the launch takes place.
North Korea has said it will send a satellite into space between April 4 and 8. It says it has the right to do so as a part of a peaceful space programme.
South Korea and Japan say the launch is a disguised test of the long-range Taepodong-2 missile, which is designed to carry a warhead capable of reaching U.S. territory but blew apart shortly after launch during its only test flight in 2006.
The United States says the launch would violate a U.N. Security Council resolution which demands that North Korea suspend all activities related to its ballistic missile program.
He also said the health of North Korean leader Kim Jong-il, widely thought to have suffered a stroke last year, appeared to be improving and that he looked to have a continued hold on power.
He repeated that any missile launch would be a violation of U.N. sanctions and that he wanted the international community to send a stern message to Pyongyang over the issue.
The Associated Press contributed to this report.