North Korea has declared a no-sail zone for its ships off its east coast, leading to worries of more missile launches days before the U.S. defense head is set to visit South Korea, local media reported Monday. U.S. Defense Secretary Ashton Carter is set to travel to South Korea for a three-day summit starting Thursday.

Pyongyang fired four short-range missiles off its western coast on Friday that South Korea claimed was an attempt to stir tensions during its annual joint military drills with the U.S.

The countries said the exercises were for defense training, but Pyongyang has accused them of rehearsing for a planned invasion. The gesture also comes close to the 103rd birthday of North Korea’s founding leader Kim Il-sung, who remains highly revered in the country.

However, a South Korean military official said Pyongyang had not announced the no-sail zone to international parties. "North Korea has not notified the International Maritime Organization and neighboring countries of its declaration of the no-fly, no-sail zone," Nah Seung-yong told reporters Monday, according to the Korea Herald. "The North might have done that internally for the safety of its people on the East Coast."

North Korea has previously informed the international community before missile launches.

Anonymous government officials told South Korean news agency Yonhap that the no-sail warning had come into effect on April 1, and could herald possible tests of the Rodong medium range missile, Reuters reported. North Korea had last tested the Rodong in March 2014, which was also near the time of a joint meeting between South Korea, Japan and the United States to discuss the North’s threatening gestures.

Experts said the mid-range Rodong missile, which has an effective range of 1,300 km (800 miles), would be the most likely candidate, according to Reuters. North Korea has threatened to hold a fourth nuclear test, which would reportedly bring it close to having a functional nuclear weapon.