South Korean navy on Thursday staged a major anti-submarine exercise. This is seen as its first show of strength since tensions with North Korea flared over the sinking of one of Seoul's warships. <br/> <br/>Yonhap news agency quoting military officials reports, about 10 ships took part in the one-day drill, which included the dropping of depth charges and naval gunfire. <br/> <br/>The drill was held off the west coast town of Taean, far south of the disputed Yellow Sea border where the Cheonan corvette was torn in two on March 26.<br/>Tensions have risen sharply since a multinational investigation concluded last week that a North Korean submarine fired a heavy torpedo to sink the Cheonan with the loss of 46 crew members. <br/> <br/>Officials said South Korea's military and the 28,500 US troops in the South had raised their alert level and stepped up surveillance of the North from satellites and reconnaissance aircraft. <br/> <br/>South Korea and the United States have also launched a diplomatic drive for United Nations Security Council sanctions. The North says the South faked evidence of its involvement in the sinking to fuel confrontation for domestic political reasons. It threatens all-out war against any punitive moves. The regime has said it is breaking all cross-border links in protest at Seoul's smear campaign. <br/> <br/>China, wielding veto power in the council, has held back so far from condemning its ally the North Korea. Russia, which also has a Security Council veto, said it would send experts to Seoul to study the findings of the investigation into the sinking. <br/> <br/>The Association of Southeast Asia Nations and the European Union in a joint statement have expressed deep concern over rising tensions in the Korean peninsula and called on both sides to exercise restraint.<br/>