Pyongyang fired a pair unspecified ballistic missiles towards the Sea of Japan, also known as the East Sea, the South Korean military said on Sunday.

South Korean news agency Yonhap cited the Joint Chiefs of Staff (JCS) as announcing the tests, held about 50 minutes apart, but no further details were released.

South Korea’s military had strengthened “our monitoring and surveillance activities” and maintained “a full readiness posture in close cooperation with the United States,” the JCS told Yonhap.

On Thursday North Korea tested a “high-thrust solid-fuel” engine that could allow for quicker launches of ballistic missiles, state media reported.

UN resolutions prohibit North Korea from testing ballistic missiles of any range.

The situation on the Korean peninsula is currently very tense. Nuclear-armed North Korea has conducted a string of missile tests recently.

Observers fear that Pyongyang’s first nuclear test in years is imminent.

Source: dpa/MIA