North Korea has increased activities at its underground nuclear site, military sources said Monday, in what could be preparations for another atomic test ahead of U.S. President Barack Obama’s upcoming Asia trip.

“The Punggye-ri nuclear test site has shown increased movement of vehicles and forces compared to the past,” a senior military official said, asking for anonymity. “South Korean and U.S. forces have been closely monitoring the latest development to detect signs of another test.”

The latest move comes as Obama is set to visit South Korea from Friday to Saturday as part of his Asia trip, which will also include stops in Japan, Malaysia and the Philippines. Earlier this month, Pyongyang threatened to conduct a “new form” of nuclear test in protest of ongoing joint South Korea-U.S. exercises. The annual drills ended on Friday.

On Monday, North Korea’s Foreign Ministry warned that Obama’s trip to South Korea could “escalate confrontation and bring the dark clouds of a nuclear arms race.”

The communist North conducted three nuclear tests in 2006, 2009 and 2013 in an apparent effort to make a warhead small enough to fit on a missile. Many experts, however, estimate the North has not yet mastered the miniaturization technology.

Seoul officials have said another test could be conducted at the southern side of the underground site at any time, though it depends on a political decision by young leader Kim Jong-un.

“While increased movement of vehicles could be one of the early signs of an atomic test, signs of an imminent test have not been detected yet,” said another military official, who spoke on condition of anonymity.

Shortly before the test in February 2013, the North set up measuring equipment and communication cables and sealed up the blast tunnel at the sprawling test site. (Yonhap)