SEOUL (Reuters) - A 5.3 magnitude seismic event in North Korea on Friday, thought to have been the North’s fifth nuclear test, would indicate a device with a 20- to 30-kilotonne yield, an analyst said, the largest yield estimate yet of a North Korean nuclear device.

“That’s the largest DPRK test to date, 20-30kt, at least. Not a happy day,” Jeffrey Lewis of the California-based Middlebury Institute of International Studies told Reuters, using the North’s official title of the Democratic People’s Republic of Korea.

“Yield estimates are always kind of approximate. The point is that it is the biggest one to date unless they revise the yield downward,” he said.

Initial analysis of North Korea’s January 2016 test estimated a yield of just 6 kilotons.