The Taepodong-2 weapon, thought to be capable of reaching the US, crashed 42 seconds after it was launched.

Washington said North Korea had fired at least six missiles, the rest of which were short-range, and could be preparing to launch others.

The UN Security Council will hold an emergency meeting to discuss the tests.

The closed session meeting, to take place on Wednesday, was requested by Japan's UN ambassador Kenzo Oshima.

Earlier, the US ambassador to the UN, John Bolton, said he was "urgently consulting" with other Security Council members on the issue.

A US state department official called the launch of the Taepodong-2 a "provocative act designed to get attention".

"The North Koreans have again clearly isolated themselves," White House press secretary Tony Snow told reporters.

He said President George W Bush had consulted Defence Secretary Donald Rumsfeld, Secretary of State Condoleezza Rice and National Security Adviser Stephen Hadley.

The Assistant Secretary of State Christopher Hill is set to fly out to the region later on Wednesday.

Mr Hadley would be meeting his South Korean counterpart in Washington, in a meeting which had already been scheduled, Mr Snow said.

Japan's Chief Cabinet Secretary Shinzo Abe said Tokyo "strongly protested" the move by North Korea.

"This is a grave problem in terms of peace and stability not only of Japan but also of international society," he said.

Japanese Foreign Minister Taro Aso said it was highly possible his country would impose economic sanctions on North Korea as a result.

In Seoul, the South Korean government called an emergency cabinet meeting.

Heightened alert

The short-range missiles, capable of travelling a few hundred kilometres, all fell safely into the Sea of Japan.

The first missile was launched at 0332 Japan Time (1832 GMT) and crashed into the sea 600 km (373 miles) from the Japanese mainland, the public broadcaster NHK TV said.

The second was fired at about 0400, Japanese news agency Kyodo said.

The US and North Korea's neighbours had been on heightened alert in recent days amid suspicions that Pyongyang was preparing to launch the Taepodong-2 from the Musudan-ri launch site in the north-east of the country.

The Taepodong-2 has a range of up to 6,000 km (3,730 miles), putting parts of the US within striking distance.

'Strong protest'

The BBC's Charles Scanlon says North Korea has been feeling under pressure and ignored in recent months with the US refusing to negotiate on its demands over its nuclear plans.

This may be a way for North Korea to get attention and break the diplomatic log jam, our correspondent adds.

On Monday, Pyongyang warned it would launch an "annihilating" nuclear strike if its atomic facilities were pre-emptively attacked by the United States.

Washington dismissed the threat as "deeply hypothetical" and urged the North Koreans to return to six-party talks aimed at curbing the country's nuclear activities. The talks stalled last year.

North Korea had agreed with Japan to a moratorium on missile tests in 2002 and this was reaffirmed two years ago.

The last time North Korea tested a long-range missile was in 1998, when it launched a Taepodong-1 over northern Japan.