North Korea appeared to have exploded an underground nuclear weapon Tuesday, with unusual seismic activity detected near the secretive police state’s mountainous test site.

There was no immediate confirmation from Pyongyang on whether it had carried out its third nuclear test, but the tremor follows weeks of threats from the North to build up its nuclear capacity and carry out an “all-out action of high intensity.”

Following a flurry of activity at the site — where tests were previously conducted in 2006 and 2009 — North Korea recently removed most of the men and equipment from the location, an apparent sign the test was imminent, according to South Korea’s Yonhap news agency.