SEOUL, June 26 - North Korea fired three short-range projectiles into the sea Thursday in an apparent show of force ahead of an expected visit by Chinese President Xi Jinping to South Korea. The projectiles were launched from the North's eastern port of Wonsan and flew 190 kilometres (115 miles) into the Sea of Japan (East Sea), the South's defence ministry said. "We are looking into exactly what type of projectiles were fired," a ministry spokesman told AFP, adding they could be missiles or rockets. "It's still unclear if North Korea test-fired new weapons, or launched them for intentional provocation," he said, adding that South Korean troops had increased their vigilance along the border. Chinese leader Xi is expected to make a state visit to Seoul next week. It would be the first time in almost two decades for a sitting Chinese president to visit South Korea before going to North Korea. The North's young leader Kim Jong-Un has yet to be invited to Beijing. It is not unusual for North Korea to carry out such tests, which often go unreported by South Korea. In March, North Korea conducted multiple launches of short-range Scud missiles and rockets to coincide with annual joint military drills South Korea conducted with the United States. The North also test-fired two medium-range ballistic missiles that flew 650 kilometres off its east coast, prompting a stern reaction from Washington and Seoul. South Korea at the time condemned the missile launches as a "reckless provocation" but stopped short of calling for UN sanctions. Shortly after Thursday's launches, the North denounced a South Korean live-fire drill near the disputed Yellow Sea border as "a grave military provocation". The North's military, in a statement carried by the official Korean Central News Agency, threatened a retaliatory strike, insisting shells fired by South Korean troops fell into North Korean waters. "The only remaining thing is an order from our supreme commander to launch a strike," it warned. The South's Yonhap news agency quoted military officials as saying South Korean troops had fired shells in a southwestern direction on Thursday. The North does not recognise the maritime boundary, the scene of brief but bloody naval clashes in 1999, 2002 and 2009. North Korea shelled the South's frontline island of Yeonpyeong in November 2010, killing four South Koreans, briefly triggering concerns of a full-scale conflict. In May North Korea fired shells into waters near a South Korean warship on patrol south of the sea border, prompting an evacuation of residents on a nearby island.