North Korea warned Monday that an upcoming Asian tour by US President Barack Obama could escalate military tensions, as South Korean media cited intelligence reports of increased activity at the North's main nuclear test site.

Obama's April 23-29 tour includes visits to Japan and South Korea, the two main US military allies in Asia and key partners in the effort to curb Pyongyang's nuclear weapons programme.

A North Korean foreign ministry spokesman said the "reactionary and dangerous" trip would only serve to "escalate confrontation and bring dark clouds of a nuclear arms race" over the Korean peninsula.

In a statement carried by the North's official KCNA news agency, the spokesman said recent Asia visits by the US Secretaries of State and Defence had sought to demonise the North's nuclear and missile programmes and justify a growing US military presence in the region.

"It is as clear as noonday that Obama will trumpet the same thing," he said.

The last two months have been a period of elevated tensions on the Korean peninsula, as South Korea and the United States conducted annual, large-scale military exercises.

North Korea protested by test-firing dozens of short-range missiles and conducting its first mid-range missile tests for nearly four years.

On Monday, the South's Yonhap news agency, citing unidentified military sources, said increased activity had been observed at the North's underground Punggye-ri site where its three nuclear tests were carried out in 2006, 2009 and 2013.

"South Korean and US forces have been closely monitoring the latest development to detect signs of another test," one source told Yonhap.

At the end of last month, North Korea said it would not rule out a "new form" of nuclear test after the UN Security Council condemned its latest medium-range missile launches.

Experts saw this as a possible reference to testing a uranium-based device or a miniaturised warhead small enough to fit on a ballistic missile.