The launch of anti-ship missiles from Wonsan, a naval base on the North's eastern coast, coincided with the arrival of U.S. Deputy Secretary of State Tony Blinken in Seoul ahead of yearly joint military exercises with the South.
According to Yonhap news agency, Unification Ministry spokesman Lim Byeong-cheol told reporters: “[The government] plans to deal resolutely with North Korea's provocation and also… build up mutual trust through dialogue and cooperation between the South and North."
The government did not give any indication of its plans but in the past it has increased border security at times of heightened tension.
Sunday’s missile launches into the East Sea were overseen by North Korean leader Kim Jong-un, who was pictured in official media smiling at the testing of a “new type of cutting-edge anti-ship rocket.”
They were the second series of missile tests this year.
North Korea routinely conducts weapons tests but the latest launches came amid a dispute over arrangements for a rare potential meeting between their leaders, suggested last month. The countries' leaders have only met face-to-face on two previous occasions in the past 70 years.
Last month, the North said it was willing to suspend nuclear tests if the U.S. canceled military manoeuvres with the South – an offer rejected by the White House.
North Korean news outlets often carry official threats to attack the U.S. – which recently moved to tighten sanctions against Pyongyang – and the launches repeat a pattern seen last year in response to the military alliance between Seoul and Washington.
Blinken is in South Korea to discuss the North’s ambitions as well as the role of Russia in the region.
South Korean Foreign Minister Yun Byung-se met his Russian counterpart Sergey Lavrov in Germany over the weekend and Moscow is seen as a key intermediary between the two Koreas, who remain technically at war as they did not sign a peace treaty after the 1950-53 conflict.