Pyongyang has set a no-sail and no-fly zone in the East Sea since last Wednesday, a government source said Monday, raising concerns over possible North Korean provocations including ballistic missile launches.

“From April 1, the North set a national alert period (no-sail/no-fly period),” said the source, declining to be identified. “It has been set since April 1, but it did not specify when the period ends.”

Military experts raised the possibility of Pyongyang launching Rodong missiles with ranges of 300-500 km.

North Korea observers have raised concerns that Pyongyang could conduct saber-rattling moves or provocations in time for the “Day of the Sun,” or the birthday of its national founder Kim Il-sung on April 15.

They also noted that Pyongyang could take some measures in a show of force or to get international attention, particularly when new U.S. Defense Secretary Ashton Carter visits Seoul from Thursday through Saturday.

Its zoning of the East Sea area appeared to be aimed at easing international criticism that it had launched missiles without prior warning. Tokyo, in particular, strongly protested the North’s launch on March 2 of two Scud ballistic missiles.

Pyongyang has recently hardened its rhetoric against the U.S. and South Korea in an apparent show of displeasure over the prolonged deadlock in its relations with both sides.

Some experts presumed that amid its continued isolation, the communist regime could carry out military provocations including conducting a fourth nuclear test and launching a long-range rocket to bring itself back into the international spotlight.

Pyongyang’s continued pursuit of nuclear arms was the main cause of its diplomatic impasse with the international community.

The North has recently said it would not engage in any denuclearization negotiations until the rest of the world including the U.S. is denuclearized. It has also reiterated that it would hold fast to its so-called “byungjin line,” a two-pronged policy of simultaneously pursuing the development of nuclear arms and its economy.

By Song Sang-ho (sshluck@heraldcorp.com)