North Korea could undertake provocations, including not only weapons tests, such as missile launches, but also covert attacks on South Korea, next month when the South and the U.S. conduct joint military exercises, a U.S. expert said Tuesday.

The communist nation is upset by the U.S. imposition of sanctions on leader Kim Jong-un last week for his role in the country's human rights violations, as well as by a decision by Seoul and Washington to deploy a THAAD missile defense battery in the South.

Pyongyang called the sanctions on Kim "an open declaration of war" and vowed to deal with all issues between it and the U.S., including the issue of two detained American citizens, according to its wartime law. It also vowed to "totally cut off" the only dialogue channel with the U.S. via its U.N. mission, called the "New York channel."

The North's military also threatened to take "physical" action against a THAAD deployment.

"The upcoming August exercise will provide North Korea with a justification for acting out," Ken Gause, a senior North Korea analyst at CNA Corp. in Washington, told Yonhap News Agency.

"The question is whether North Korea will stick with demonstrations and tests or will it try something more aggressive.

If North Korea tries something aggressive, it most likely will be a covert -- versus an overt -- attack," he said.

The North has protested angrily every time the South and the U.S. conduct annual drills, calling such maneuvers a rehearsal for invasion, despite repeated assurances from Seoul and Washington that the exercises are purely defensive.

Gause said the North's shutdown of the "New York channel" is no surprise.

"North Korea is in no mood to discuss anything with the United States. They have come to the conclusion that any meaningful headway on issues of importance to Pyongyang cannot be made under the current administration, which has taken the unprecedented step of sanctioning its supreme leader," he said.