North Korea's Foreign Ministry has said that forcing it to give up nuclear weapons is tantamount to a declaration of war, coinciding with the launch of cruise missiles off its east coast.
South Korea's Yonhap news agency reported that North Korea fired several cruise missiles, and that the South Korean military detected the launch from the Ham Hong area, on North Korea's east coast.
Pyongyang's latest missile launch on Wednesday comes just three days after it fired a short-range ballistic missile into the sea off its east coast.
South Korea's Yonhap news agency said Wednesday's launches may have included cruise missiles, which are strategic weapons for North Korea and the word "strategic" is commonly used to describe weapons with nuclear capabilities.
escalation
North Korea has stepped up military tests in recent weeks, launching an intercontinental ballistic missile last week and conducting what it described as a simulation of a nuclear counterattack on the United States and South Korea over the weekend.
South Korea's Joint Chiefs of Staff said the military was on high alert and that South Korean and U.S. intelligence services were analysing missile details.
The U.S. amphibious assault ship McCain docked in South Korea on Wednesday for the first large-scale amphibious landing exercises for the two allies in five years, the U.S. military said.
South Korea's military said in a statement: "We will successfully conclude the Freedom Shield exercise as scheduled under a strong joint defense posture."
Seoul and Washington are due to conclude an 11-day drill dubbed Freedom Shield 23 on Thursday, which Pyongyang has described as preparations for a "war of aggression" against it.