The Defense Ministry on Friday refuted U.S. media reports that Washington was aware of Pyongyang’s preparations in the run up to Wednesday’s fourth nuclear test.

NBC News reported Wednesday, Washington time, that the U.S. had perceived the movement for two weeks and launched drones to collect air samples near the site, citing a senior military official.

“The report appears to be based on assumptions,” a ministry official told reporters on customary condition of anonymity. “Normally a U.S. drone cannot enter North Korea’s airspace. One may be able to fly over the East Sea, but it must be for reconnaissance activities, not air sampling.”

The report came after Seoul has come under fire for its failure to spot any signs of the experiment in contrast to a top defense intelligence official’s earlier remark during a parliament audit last September that it is capable of foreseeing an upcoming explosion up to a month beforehand.

Under a trilateral arrangement inked last year, Seoul, Washington and Tokyo are supposed to exchange military intelligence on North Korea’s nuclear and missile activities.

As lawmakers poured out criticism, the Joint Chiefs off Staff’s intelligence head said Wednesday that the preparations had taken place so clandestinely that U.S. Forces Korea commander Gen. Curtis Scaparrotti did not acknowledge in advance. National Intelligence Service officials also argued during the session that not only South Korea but also other countries had not detected such signs.

By Shin Hyon-hee (heeshin@heraldcorp.com)