WONSAN, NORTH KOREA — South Korea says the North has launched missiles into the sea, amid growing uncertainty about nuclear negotiations between Washington and Pyongyang.

According to South Korea's Joint Chiefs of Staff, North Korea fired two unidentified projectiles from Wonsan on Thursday, at 5:34 and 5:57 a.m.

Each flew at least 430 kilometers, at an altitude of 50 kilometers, before landing in the East Sea.

Based on the flight range, the projectiles are believed to be the North Korean version of Russia's Iskander ballistic missile. They resembles the two short-range missiles Pyongyang fired in May.

Vipin Narang, an associate professor of political science at MIT, told CNN the launch may be a response to the news that joint U.S.-South Korea military exercises will go ahead as planned next month, despite objections from North Korea.

CNN reports that the launch came just days after North Korean state media released a photo of leader Kim Jong-un inspecting a submarine under construction, in what may be an attempt to showcase Pyongyang's military capabilities.