North Korea fired more than 10 short-range ballistic missiles toward the waters off its eastern coast on Thursday, escalating tensions on the Korean Peninsula as joint US-South Korea military exercises approach their conclusion.

South Korea’s presidential office convened an emergency security meeting following the launches and ordered the military to maintain full readiness. Seoul condemned the missile launches as a serious violation of United Nations Security Council resolutions.

South Korea’s Joint Chiefs of Staff said the missiles were launched from the Pyongyang area and travelled about 300 kilometres before falling into the sea. Japan also reported that the projectiles landed outside its exclusive economic zone, according to NHK.

South Korean Defence Minister Ahn Gyu-back told lawmakers that the weapons were believed to involve North Korea’s 600mm multiple rocket-launcher system, known externally as the KN-25. Seoul has been monitoring the system because of concerns that it could potentially be deployed near the inter-Korean border.

North Korea has previously claimed that its 600mm system is capable of carrying tactical nuclear warheads. Ahn also said North Korea is believed to possess between 80 and 120 nuclear warheads.

The latest launches came shortly after North Korean leader Kim Jong Un’s sister, Kim Yo Jong, criticised the ongoing US-South Korea military exercises. Her comments followed President Donald Trump’s decision to substantially reduce US participation in the drills and his stated interest in restarting diplomacy with Kim Jong Un.

Trump said earlier that he had received a response from the North Korean leader. However, Kim Yo Jong said she was unaware of any recent communication between the two leaders and maintained that Washington’s “hostile policy” toward Pyongyang had not changed.

Yang Moo-jin, a professor at the University of North Korean Studies in Seoul, said the missile launch appeared intended to demonstrate that Kim Yo Jong’s warnings were serious. He suggested that Pyongyang may have been protesting the military exercises, objecting to discussions about its nuclear arsenal and testing improved weapons systems.

The launch also came as Chinese Foreign Minister Wang Yi visited Seoul. Wang urged South Korea not to take sides between Beijing and Washington and called on the United States to abandon what China describes as a hostile policy toward North Korea.

With military exercises ending Friday, the latest missile activity highlights the difficulty of balancing reduced military tensions with efforts to revive diplomatic engagement with Pyongyang.