The United States has upped its watch of North Korea after Pyongyang declared on Tuesday a no-sail zone in the waters south of the Northern Limit Line (NLL) in the Yellow Sea.

U.S. Assistant Secretary of State Philip Crowley said Wednesday that Washington is studying the possible reasons for the North’s recent move. He said that in any kind of declaration like the recent one, the U.S. would encourage restraint on both Koreas.

Earlier on Tuesday, a military official in Seoul said the North ordered ships to stay away from an area north of Baengnyeong Island in the Yellow Sea and another area east of the South Korean island from January 25th until March 29th.

The official says the government is closely monitoring the areas to find out whether the designation is just part of routine winter military exercises or if it is a sign that the North is preparing to launch short-range missiles.