Unidentified assailants fired a shell near Japan's base in southern Iraq on Tuesday, the third attack to rattle Japanese troops in as many days amid signs of deteriorating security in the region, the government said. No injuries or damage were sustained in the explosion outside the Japanese camp in the Iraqi city of Samawah early Tuesday, Chief Cabinet spokesman Hiroyuki Hosoda told reporters. The government was investigating the attack, said Hosoda. Some 500 Japanese soldiers are based in Samawah, part of a total deployment of 1,000 military personnel on a noncombat mission to purify water and repair infrastructure.