Australian Prime Minister Malcolm Turnbull has said, counter-terrorism police have stopped a suspected plot to bring down an aeroplane. He said four people were arrested in raids across Sydney.Investigators said they have seized materials in the raids that could have been used to make an improvised explosive device.<br/><br/>Mr Turnbull said the raids have been a major joint counter-terrorism operation. He said extra security is in place at domestic and international airports. State broadcaster ABC reported that the raids took place in the Sydney suburbs of Surry Hills, Lakemba, Wiley Park and Punchbowl.