US submarine has arrived in South Korea, amid worries of another North Korean missile or nuclear test. <br/><br/>The missile-armed USS Michigan is set to join an incoming group of warships led by aircraft carrier Carl Vinson. North Korea is celebrating its army's 85th founding anniversary today. <br/><br/>It has previously marked similar occasions with missile tests. Tensions have risen on the Korean peninsula in recent weeks as the US and North Korea exchange heated rhetoric.<br/><br/>Meanwhile in an unusual event, the entire US Senate has been asked to attend a briefing on North Korea tomorrow at the White House.