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Etihad Airways has said it is assisting Australian police with their investigation into a suspected terrorism plot to bring down a plane.

Police have been across Sydney after four men were on Saturday.

Investigators have said the alleged plan most likely involved detonating an improvised explosive device.

Police did not name the airline involved, but officials have described the operation as "sophisticated".

"The Etihad Airways aviation security team is assisting the Australian Federal Police with its investigation and the matter is ongoing," the airline said in a brief statement on Tuesday.

"Etihad is complying fully with the enhanced security measures at airports in Australia and monitoring the situation closely." It did not give further details.

Local media reported the men planned to use poisonous gas or a crude bomb disguised as a meat mincer, but police have not confirmed details of the alleged plot.

Australian PM Malcolm Turnbull has said the raids had been a "major joint counter-terrorism operation".

On Sunday, a magistrate gave permission for an additional period of detention, meaning the four can be held for up to seven days without charge.

Police said no charges have been laid.

Australian airports have increased their security measures and passengers were left waiting in lengthy queues after being advised to arrive earlier than usual.

The national terror threat level remains at "probable".