The blast in Yanggang Province, close to the Chinese border, is said to have happened on Thursday as the Stalinist state celebrated its National Day.
Western officials have downplayed suggestions it was a nuclear test.
As the North keeps silent on the blast, a visiting UK minister has asked for an urgent explanation.
The area is mountainous and thinly populated, and home to a major military base, according to reports.
On Saturday, the North’s foreign ministry said reports of nuclear experiments in the South made it even more determined to pursue its own programme.
