North Korea fired multiple ballistic missiles off its east coast, Seoul’s military said on Monday, after weeks of mounting friction over a landmine explosion along the heavily fortified border that divides both countries.Since the blast that injured three South Korean soldiers in the Seoul-controlled area of the Demilitarised Zone (DMZ), both countries have touted the test-firing of upgraded missile technology.”Our military detected around 09:20 (0220 CET) several ballistic missiles launched from the Pyongyang area towards the East Sea,” the Joint Chiefs of Staff (JCS) said, referring to the body of water also known as the Sea of Japan.”Our military has stepped up surveillance and vigilance against possible additional launches,” it added in its statement.Japan’s defence ministry separately confirmed the launch of at least one suspected ballistic missile.Some of the North Korean missiles detected flew more than 900 kilometres, the JCS said, while Seoul and longtime ally the United States were now conducting an analysis of their exact specifications.South Korea’s National Security Office convened an emergency security meeting following the launch to review the country’s military readiness.A joint Seoul-UN investigation last week concluded that North Korea had planted the landmines that exploded in the DMZ over a year ago, something Pyongyang has angrily denied.Kim Yo-jong, the powerful sister of North Korean leader Kim Jong-un, has blasted South Korean demands for an apology as “despicable” while separately rejecting an offer of medical equipment from Seoul.Duelling testsOn 3 October, Pyongyang test-fired an intermediate-range ballistic missile that Kim Yo-jong said employed unspecified artificial intelligence technology and was “capable of changing its flight orbit at a low altitude.”Seoul said days later that it had successfully tested a domestically produced hypersonic glide vehicle (HGV), a weapon carried into the upper atmosphere by rocket boosters or ballistic missiles before gliding towards its targets.The presidential Blue House said the HGV offered “a new deterrence capability capable of striking key targets within the Korean Peninsula’s operational environment.”On Thursday, Kim Yo-jong said Seoul must offer “utmost respect” if it hoped to mend ties with the North.”If they (South Korea) truly wish to restore relations between the DPRK and South Korea, they should be mindful of the other side’s feelings in every word and deed and first demonstrate the utmost respect, consideration and sincerity,” she said Thursday, using her country’s official acronym.The rising tensions come as US President Donald Trump’s recent overtures to Pyongyang have raised concerns that Seoul, an ally of Washington, could be sidelined in potential nuclear talks involving the North.In October, North Korea launched several short-range ballistic missiles a day after Trump suggested he might meet with Kim Jong-un this year.Seoul and Pyongyang remain technically at war as the Korean War ended in 1953 with a truce rather than a peace treaty.The DMZ is one of the world’s most heavily fortified frontiers and landmine incidents have previously heightened tensions between the two countries.