By Ju-min Park
SEOUL (Reuters) - North Korea attempted and failed to launch what experts believe was an intermediate-range ballistic missile on Friday in defiance of U.N. sanctions and in an embarrassing setback for leader Kim Jong Un, drawing criticism from major ally China.
The U.S. Defense Department called the test of the road-mobile missile, which came as North Korea celebrated the birthday of Kim's grandfather and North Korea's founding father Kim Il Sung, a "fiery, catastrophic" failure.
The test, on North Korea's so-called "Day of the Sun," followed its fourth nuclear test in January and a long-range rocket launch in February, which led to new U.N. sanctions that have failed to halt Pyongyang's weapons programs.
South Korean officials and international experts said the failed test heightened the possibility that North Korea would conduct a fifth nuclear test, possibly within weeks.
The U.S.-based 38 North website, which specializes in North Korea, said there has been activity at the country's nuclear site based on satellite imagery and on Wednesday said the possibility of a fifth nuclear test "could not be ruled out."
U.S. officials said the missile tested on Friday never got off the launch pad but further tests were expected.
"We're still assessing the specifics of it but I can tell you that it was a fiery, catastrophic attempt at a launch," Pentagon spokesman Captain Jeff Davis told a briefing. "It was not successful."
Davis said the U.S. military thought the device was a road-mobile missile because the launch occurred at a coastal site where Pyongyang ordinarily does not test rockets.
He said North Korea, which regularly threatens to destroy South Korea and the United States and often fires missiles during periods of tension, remained a security concern in spite of the failure.
"We know that this is a capability that they are aiming towards," he said. It's ... a missile system that they've displayed on multiple occasions and that is why we have a ballistic missile defense system that we have invested in very heavily to be able to outpace that threat as that threat develops further."
The White House strongly condemned the latest launch attempt and said Washington would work with China and other countries to put pressure on North Korea, which faced the prospect of further isolation.
China, North Korea's most important economic and diplomatic backer, has been angered by Pyongyang's nuclear tests and rocket launches and has backed U.N. sanctions.
"The firing of a mid-range ballistic missile on Friday by the Democratic People's Republic of Korea (DPRK), though failed, marks the latest in a string of saber-rattling that, if unchecked, will lead the country to nowhere," China's official Xinhua news agency said in an English language commentary.
"... Nuclear weapons will not make Pyongyang safer. On the contrary, its costly military endeavors will keep on suffocating its economy."
Chinese Foreign Ministry spokesman Lu Kang said the U.N. Security Council was clear on North Korean rocket launches.
"At present, the situation on the peninsula is complex and sensitive," he told reporters. "We hope all parties can strictly respect the decisions of the Security Council and avoid taking any steps that could further worsen tensions."
In 2012, Kim Il Sung's birthday was also marked by an attempted long-range rocket launch, which also failed.
The missile was likely a Musudan, South Korea's Yonhap news agency said, an intermediate-range ballistic missile with a design range of more than 3,000 km (1,800 miles) that can be fired from a road mobile launcher but which has never been flight-tested.
Experts see North Korea's Musudan test as part of an effort to develop an intercontinental ballistic missile that can reach the mainland United States. The Musudan theoretically could reach the U.S. Pacific island of Guam.
North Korea is to hold a congress of its ruling Workers Party - the first such meeting in 36 years - in early May, at which Kim Jong Un is likely to trumpet his achievements in building up Pyongyang's weapons prowess.
South Korean officials and experts say he will be keen to go into that with a show of strength, not a failed rocket launch, making the possibility of another nuclear test more likely.
In a new report on Friday, 38 North said it had seen fresh activity in satellite images of North Korea's main nuclear complex, suggesting that reprocessing may be under way to produce more plutonium for atomic bombs.[L5N17I4IM]
38 North predicted last year that North Korea's nuclear weapons stockpile could grow to 20, 50 or 100 bombs within five years, from an estimated 10 to 16 weapons at that time.
(Additional reporting by James Pearson in Seoul and David Brunnstrom, Mark Hosenball and Roberta Rampton in Washington; Editing by Nick Macfie and Bill Trott)