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South Korea said Tuesday that it has detected increased activity at North Korea's main nuclear test site, CNN reported.

"We confirm that we have spotted several activities related to the nuclear test in Punggye-ri in North Korea," the South Korean Defense Ministry said in a statement. It didn't specify what the activities were.

The ministry said it had stepped up its military preparedness since Monday morning and was monitoring around the clock for signs of activity at the site.

North Korea said last month that it wouldn't rule "a new form of a nuclear test" to strengthen its nuclear deterrent. Experts have speculated that that could refer to the testing of a uranium bomb.

The reclusive regime in Pyongyang is known to have conducted three previous tests, all of them believed to be based on plutonium. The most recent one took place February 2013.

The past tests prompted international condemnation and the imposition of sanctions aimed at hampering the North's weapons program.

Since the last underground detonation, South Korean officials have repeatedly said that they believe North Korea is ready to carry out another test at any time, pending a political decision to go ahead with it.