North Korea has reportedly launched a long-range rocket, defying calls from world leaders.
Japanese television broadcaster NHK and South Korea's Yonhap news agency reported the launch on Sunday.
The US later confirmed the launch. The UN Security Council is to hold an emergency session on the development later in the day.
"We have had a launch. I don't know the type of missile," Fred Lash, a US state department spokesman, told reporters.
The rocket reportedly passed over Japan on its way towards the Pacific Ocean.
Pyonyang has yet to confirm the firing of the rocket.
The launch, which Pyongyang has said is intended to send a communications satellite into orbit as part of a peaceful space program, has sparked alarm because Noth Korea has acknowledged it has nuclear weapons.
Lash called the move a "provocative act" and said the US would "take appropriate steps to let North Korea know that it can't threaten the safety and security of other countries with impunity and acts like these."
"North's Korea's development, deployment and proliferation of missiles, ballistic missile-related materials, equipment and technologies pose a serious threat to the northeast Asia region and to the international community," he said.
'Reckless'
South Korea said that the launch was "reckless".
"The government cannot but express disappointment and regret over North Korea's reckless act of firing a long-range rocket, which poses a serious threat to security on the Korean peninsula and the world," Lee Dong-Kwan, a South Korean presidential spokesman, said.
"The government will deal firmly and resolutely with North Korea's provocative act," Lee said without giving further details.
N Korea's 'space program'
North Korea says it launched its first satellite, Kwangmyongsong-1 (right), into orbit aboard a Taepodong 1 rocket in 1998
It says the satellite launch was successful, beaming a looped recording of the Song of General Kim Il Sung back to Earth
US space command said at the time it was unable to find any North Korean satellite in orbit
North Korea says it is now preparing to launch the Kwangmyongsong-2 satellite on top of what it has called an Unha-2 rocket
"It is extremely regrettable that North Korea went ahead with the launch ... and we protest strongly," Takeo Kawamura, Japan's chief cabinet secretary, said.
"It has flown over Japan so no orders have been issued to intercept or destroy the projectile," he said.
Tensions over the launch have run high. On Saturday, the Japanese government, reported that North Korea had launched the rocket but later retracted the news, blaming a faulty detection device for the erroneous information.
The South Korean and Japanese governments allege that North Korea is using the launch to test its Taepodong-2 long-range missile, which is capable of reaching the US.
Wayne Hay, Al Jazeera's correspondent in Paju on the South Korean side of the border with the North, said that although it was considered to be a satellite launch, South Korea remained cautious.
"I don't think that in this particular launch they would be worried. But [South Korea] have stuck to the line that this is a cover for a ballistic missile test," he said.
Our correspondent also said that South Korea has ordered 655,000 troops to be on high alert to guard against any provocative activity.
'Strong messages'
Leonid Petrov, a research associate at the School of Pacific and Asian Studies at the Australian National University, said that the launch had been successful.
"It looks like everything is under control and North Korea and there was a significant development and improvement into the technology ... because previous launches either failed or there was a controlled explosion 42 seconds after launch," he said.
Petrov said the launch sent two strong messages: "One to enemies ... That North Korea has a new technology for deterrence. So nobody should interfere into North Korean domestic affairs otherwise there will be retaliation.
"[And one] to friends ... that North Korea is possessing a superior technology not only in satellite launching but also as a duel technology in missile launching," Petrov said.
"So North Korea can export more missiles and gain more revenue."
South Korea, the US and Japan said earlier in the week they would take North Korea to the UN security council if it went ahead with the launch.
PHOTO CAPTION
Japan Ground Self-Defense Force (JGSDF) soldiers react following North Korea's launch of a rocket, at JGSDF Araya base, where Patriot Advanced Capability-3 (PAC-3) missiles are deployed, in Akita, northern Japan April 5, 2009.
Al-Jazeera