No excessive radioactivity was detected in South Korea after North Korea announced it had successfully carried out an underground nuclear test on Monday, experts said.
"No radioactivity has yet been detected from the alleged nuclear test," said Han Seung-Jae, director of the state-run Nuclear Emergency Preparedness Department (NEPD).
"It might not be detected at all if the alleged nuclear testing was conducted in a tightly sealed atmosphere such as a deep tunnel, and all radioactive rays and fallout are contained," he said.
The NEPD operates 37 observation posts across South Korea to detect radioactivity, including one on the southeastern island of Ullung, 150 kilometers off the coast.
Prevailing winds might have carried radioactivity in that direction, if any had escaped from the supposed test site.
North Korea's Korean Central News Agency (KCNA) said the underground test was carried out safely and successfully and there was no radiation leak.
"Had there been a radiation leak, it would depend on the direction and strength of wind for the radioactivity to be detected here," Han said.
A South Korean defence ministry official said the test was carried out at Hwadaeri near Kilju in North Hamgyong Province on the northeast coast, at 10.36 am.
The South's presidential office said the state intelligence agency detected a 3.58 magnitude seismic tremor in that area.
The test is believed to have been conducted in a horizontal tunnel, a lawmaker quoted intelligence officials as saying.