SYDNEY - Australian Prime Minister Malcolm Turnbull said on Thursday the country's intelligence agency had downgraded the threat level to aviation after an alleged "Islamic-inspired" plot to bring down a plane was uncovered and disrupted.Four men were arrested in raids in several suburbs in Sydney, Australia's largest city, at the weekend and held without charge under special terror-related powers. One man has since been released.Increased airport security measures introduced after the arrests have caused delays for some air transport operators. It is unclear whether the security measures were designed to impede other known threats.The Australian Security Intelligence Organisation raised the threat level to aviation to "probable" as a result of the plot, Turnbull told reporters in the city of Perth on Thursday. He said it had now been downgraded to "possible.""That is because the plot has been disrupted and has been contained," said Turnbull, adding charges would be laid in due course, without elaborating.The country's two biggest domestic airlines, Qantas Airways and Virgin Australia, did not immediately respond to questions concerning the changed threat level.Police have not released details of the plot, other than it was to include an "improvised device." A US official familiar with the arrests told Reuters that the target appeared to have been a commercial flight from Sydney to the Gulf.Abu Dhabi's Etihad Airways said on Tuesday it was assisting the Australian Federal Police in the investigation.