North Korea fired ballistic missiles off its east coast on Friday, South Korea’s military said, a day after South Korea and the US flew powerful fighter jets for a joint drill that the North views as a major security threat.

South Korea’s Joint Chiefs of Staff said the launches were made from the North’s east coast Wonsan region around 3.10 p.m. It said South Korea’s military had bolstered its surveillance posture and was maintaining readiness.

North Korea in recent months has maintained an accelerated pace in weapons testing as it continues to expand its military capabilities while diplomacy with the United States and South Korea remained stalled.

Observers say North Korea likely believes an upgraded weapons arsenal would give it leverage to win greater concessions from the US if negotiations resume.

Last week, North Korean leader Kim Jong Un supervised another test firing of a new multiple rocket launch system, according to the North’s state media.

North Korea has maintained it was forced to boost its nuclear and missile programmes to deal with US-led hostility.

North Korea cites the expanded US-South Korean military training that it calls invasion rehearsals.

On Thursday, two South Korean F-35As and two US F-22 Raptors were mobilised for combined aerial exercises over the central region of South Korea. North Korea is extremely sensitive to the deployment of sophisticated US aircraft.