SEOUL, Feb 7 (Reuters) - North Korea launched a long-range rocket on Sunday carrying what it called a satellite, but its neighbours and the United States denounced the launch as a missile test, conducted in defiance of U.N. sanctions and just weeks after a nuclear bomb test.

The U.S. Strategic Command said it had detected a missile entering space, and South Korea's military said the rocket had put an object into orbit.

North Korea said the launch of the satellite Kwangmyongsong-4, named after late leader Kim Jong Il, was a "complete success" and it was making a polar orbit of Earth every 94 minutes. The launch order was given by his son, leader Kim Jong Un, who is believed to be 33 years old.

The launch prompted South Korea and the United States to announce that they would explore the feasibility of deploying an advanced missile defence system in South Korea, which China and Russia both oppose, "at the earliest possible date."

North Korea's state news agency carried a still picture of a white rocket that closely resembled a previously launched rocket, lifting off. Another showed Kim surrounded by cheering military officials at what appeared to be a command centre.

North Korea's last long-range rocket launch, in 2012, put what it called a communications satellite into orbit, but no signal has ever been detected from it.

"If it can communicate with the Kwangmyongsong-4, North Korea will learn about operating a satellite in space," said David Wright, co-director and senior scientist at the Global Security Program of the Union of Concerned Scientists.

"Even if not, it gained experience with launching and learned more about the reliability of its rocket systems."

The rocket lifted off at around 9:30 a.m. Seoul time (0030 GMT) on a southward trajectory, as planned. Japan's Fuji Television Network showed a streak of light heading into the sky, taken from a camera at China's border with North Korea.

North Korea had notified U.N. agencies that it planned to launch a rocket carrying an Earth observation satellite, triggering opposition from governments that see it as a long-range missile test.

The U.N. Security Council will hold an emergency meeting on Sunday to discuss the launch, at the request of the United States, Japan and South Korea, diplomats said.

Isolated North Korea had initially given a Feb. 8-25 time frame for the launch but on Saturday changed that to Feb. 7-14, apparently taking advantage of clear weather on Sunday.

North Korea's National Aerospace Development Administration called the launch "an epochal event in developing the country's science, technology, economy and defence capability by legitimately exercising the right to use space for independent and peaceful purposes".

The launch and the Jan. 6 nuclear test are seen as efforts by the North's young leader to bolster his domestic legitimacy ahead of a ruling party congress in May, the first since 1980.

North Korea's embassy in Moscow said in a statement the country would continue to launch rockets carrying satellites, according to Russia's Interfax news agency.

NEW MISSILE DEFENCE?

South Korea and the United States said that if the advanced missile-defence system called Terminal High Altitude Area Defence (THAAD) was deployed to South Korea, it would be focused only on North Korea.

South Korea had been reluctant to discuss openly the possibility of deploying THAAD.

"North Korea continues to develop their nuclear weapons and ballistic missile programs, and it is the responsibility of our Alliance to maintain a strong defense against those threats," Gen. Curtis M. Scaparrotti, U.S. Forces Korea commander, said in a statement. "THAAD would add an important capability in a layered and effective missile defense."

China, South Korea's biggest trading partner, repeated what it says is "deep concern" about a system whose radar could penetrate its territory.

South Korea's military said it would make annual military exercises with U.S. forces "the most cutting-edge and the biggest" this year. North Korea objects to the drills as a prelude to war by a United States it says is bent on toppling the Pyongyang regime.

The United States has about 28,500 troops in South Korea.

U.S. Secretary of State John Kerry said the United States would work with the U.N. Security Council on "significant measures" to hold North Korea to account for what he called a flagrant violation of U.N. resolutions on North Korea's use of ballistic missile technology.

South Korea's navy retrieved what it believes to be a fairing used to protect the satellite on its journey into a space, a sign that it is looking for parts of the discarded rocket for clues into the isolated North's rocket programme, which it did following the previous launch.

China expressed regret over the launch and called on all sides to act cautiously and refrain from steps that might raise tension. China is North Korea's main ally, although it disapproves of its nuclear weapons programme.

Russia, which has in recent years forged closer ties with North Korea, said the launch could not but provoke a "decisive protest", adding Pyongyang had once again demonstrated a disregard for norms of international law.

"We strongly recommend the leadership of the Democratic People's Republic of Korea think about whether a policy of opposing the entire international community meets the interests of the country," Russia's foreign ministry said in a statement.

U.N. Secretary-General Ban Ki-moon strongly condemned the launch and urged North Korea to "halt its provocative actions".

South Korean President Park Geun-hye said it was an unforgivable act of provocation.

Australia condemned what it called North Korea's dangerous conduct while Japanese Prime Minister Shinzo Abe said the launch was "absolutely unacceptable", especially after the North's nuclear test last month.

North Korea has said that its fourth nuclear test was of a hydrogen bomb. The United States and other governments have expressed doubt over that claim.

North Korea is believed to be working on miniaturising a nuclear warhead to put on a missile, but many experts say it is some way from perfecting such technology.

It has shown off two versions of a ballistic missile resembling a type tthat could reach the U.S. West Coast, but there is no evidence the missiles have been tested.