Its mission is to collect samples from the atmosphere in order to detect and identity a nuclear explosion.

It arrived at Kadena Air Base in Okinawa, Japan, early this month amid indications that the North is preparing for another nuclear test.

Thursday's flight appears meant to check whether the secretive communist nation has detonated a nuclear bomb.

It also tests operational readiness in the event of an actual provocation by the North, added the source.

There was an unconfirmed rumor that North Korea has notified China of its nuclear experiment plan.

South Korea's military said earlier the North seems all set to conduct what would be its sixth nuclear test at its Punggye-ri site in its northeastern region anytime on the leadership's decision. (Yonhap)