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| A North Korean patrol boat is seen near a large octopus shaped kite along the Yalu river near Dandong, China, Tuesday, June 2 , 2009. | ||
Daniel Pinkston, a Seoul-based analyst for the International Crisis Group, said that without knowing exactly how strong a bomb the North detonated May 25, it was impossible to say whether it could be considered a success. He was the yield was likely in the 2.5- to 8-kiloton range.
"It very well could be that they are pretty much where they want to be," Pinkston said. "They are probably trying to make a small warhead."
But efforts to assess the test were slowed by the North's rapid-fire launches of six ground-to-ship or ground-to-air missiles in the days that followed, and more launches were expected.
"It seems radioactivity-level measurements have not been that good because the North kept firing missiles," said Cha Du-hyeon, chief of the North Korean military research division at the Korea Institute for Defense Analyses.
South Korean media reported Monday that North Korea had transported its most advanced long-range missile to a launch site on its western coast near China and that it could be ready for launch in a week or two.
It was also preparing to launch several medium-range missiles, possibly modified versions of its Rodong series, from a base in the east, Yonhap news agency said Tuesday.
After North Korea's first nuclear test in 2006 it did not follow up with missile launches, but its missile activity this time has made monitors nervous, Cha said.
"Countries appear to be cautious about sending ships and aircraft to areas close to the North," he said.
The missile tests slowed efforts by South Korea, U.S. and Japanese airplanes to conduct monitoring flights aimed at detecting radiation levels that could be used to assess the strength of the test.
Japan's Defense Ministry said it has sent several T-4 fighters on surveillance missions to monitor radiation levels. The United States has RC-135s and other reconnaissance planes based on the southern Japanese island of Okinawa and more in South Korea that could be used for spying on North Korea.
"The firing of anti-aircraft and anti-ship missiles were to deter U.S. and Japanese ships and aircraft from coming close to collect evidence of the nuclear test, while at the same time threatening the South," said Kim Sung-man, a defense expert who served as a top navy official until 2005.
The intelligence gathered has not been made public.
North Korea's official Korean Central News Agency said Monday that the number of foreign spy planes operating in its airspace had risen dramatically in May.
With information still spotty, estimates of the yield of last Monday's nuclear device vary widely.
One of the first estimates of its size came from the Russian defense ministry, which put the yield at between 10 and 20 kilotons — comparable to the U.S. bombs that flattened Hiroshima and Nagasaki in Japan in August 1945.
But a senior U.S. administration official, speaking last week on condition of anonymity, said it appeared the explosive yield was much smaller, perhaps a few kilotons. The official said more technical analysis would be needed to give a more accurate reading.
In Japan, seismic experts used instruments for measuring earthquakes to estimate the power of the blast.
Gen Aoki, a quake expert and official at the Japan Meteorological Agency, said North Korea's latest nuclear test measured magnitude-5.3. The Japanese agency measured the North's 2006 nuclear test at magnitutde-4.9.
"The seismic activity of last month's nuclear test showed quake energy was four times more powerful than the previous test in 2006."
U.S. and French officials have said the 2006 test measured less than a kiloton; 1 kiloton is equal to the force produced by 1,000 tons of TNT.
Analyst Cha said it is also difficult to determine the yield of the blast because there are many factors affecting it — including what kind of a detonator was used and how tightly the underground site was sealed.
"But what is clear is that they are making progress," he said.
By Jae-soon Chang (AP)
