(Reuters) - The U.S. Geological Survey said on Saturday it could not conclusively confirm whether a magnitude 3.5 earthquake detected in North Korea was manmade or natural. USGS put the epicenter of the quake 22 km (14 miles) east-northeast of Sungjibaegam, North Korea, near the country s main nuclear test site. Chinese earthquake officials earlier said the tremor was a  suspected explosion , while South Korean officials and CTBTO, the nuclear proliferation watchdog, both said they believed it was a natural quake.  The depth is poorly constrained and has been held to 5 km by the seismologist,  USGS said.  The Air Force Technical Applications Center (AFTAC) is the sole organization in the U.S. federal government whose mission is to detect and report technical data from foreign nuclear explosions.  