North Korea steps up activity at its nuclear site, South says

North Korea has stepped up activity at its main nuclear test site, possibly preparing to carry out a fourth underground blast, South Korea said Tuesday.

The report comes just days before U.S. President Barack Obama is due to arrive in South Korea as part of a visit to several Asian countries.

"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 declined to specify what the activities were, saying the information was classified.

The ministry said it had increased its military preparedness since Monday morning and was monitoring around the clock for signs of activity at the Punggye-ri site in a northeastern region of North Korea.