Dangerous South American fire ants have invaded one of Australia's largest cities, and officials are concerned the insects will migrate farther. The fire ant has a savage bite that can kill people who suffer allergic reactions to its venom, the Queensland State Department of Primary Industries said Tuesday. The ant can also cause root damage to some crops, the department said. Ian Douglas, the department's manager of emergency response, said three sites in the state capital Brisbane have been infested with thousands of fire ants, and there have been hundreds of suspected sightings. Some people have been stung by the ants, but no deaths have been reported, he said. The ants look like normal garden variety ants, ranging in size from a tenth of an inch to around a quarter inch - but they are more aggressive than most of their cousins, Mr. Douglas said. ''If the nest is disturbed they tend to fly out very aggressively to protect the nest,'' and their bite leaves a circular pattern on the skin, Douglas is quoted by The Associated Press as saying. According to Mr. Douglas, the ants probably entered Australia by cargo ship up to two years ago. They can migrate up to 12 miles a year, he added. The fire ant originated in South America, but spread to the United States in the 1930s. It has also been discovered in south-east Asia, Central America, Papua New Guinea and West Africa.