A man instructed by the “Islamic State of Iraq and Syria” (ISIS) to bring down an Etihad Airways flight tried to use an unsuspecting passenger to carry a bomb on board, with a second poisonous gas plot also in the works, Australian police alleged Friday (August 4th).

The improvised device, using "high military-grade explosive", was due to be smuggled onto a July 15th service from Sydney, but the attempt was aborted before they reached security.

Security was tightened at domestic and international airports across Australia after four men were arrested in raids in Sydney on Saturday.

Two Lebanese-Australian men -- Khaled Khayat, 49, and Mahmoud Khayat, 32 -- have been charged with terrorism offences and were refused bail on Friday. One of the four was released without charge and another is still being questioned.

Police claim one of them planted the bomb in the passenger's luggage. Local reports said the luggage belonged to the man's own, innocent, brother.

Authorities also foiled a second alleged plot involving a "chemical dispersion device", designed to release hydrogen sulphide, but this was in the early stages.

Hydrogen sulphide is highly toxic, and often fatal for people exposed to it.

The second plan was hatched after the first one failed, police alleged, and was not necessarily targeted at a plane.

The plot was orchestrated by "a senior member of ISIS" based overseas, they said.

Police would not reveal who the ISIS link was but said it will be alleged the men were introduced to him by a relative who is a senior ISIS member in Syria.

The destination of the flight was not revealed, although reports have previously suggested it was Abu Dhabi. Etihad earlier this week confirmed it was working with authorities in Australia on their investigation.