Two men have been charged with terrorism offences after Australian police thwarted an alleged plot to blow up a plane, authorities said Thursday, as pilots warned of major airport security gaps despite efforts to tighten screenings.

Four men were arrested in Sydney on Saturday accused of planning an attack using an improvised explosive device, prompting authorities to increase security at airports across the nation.

One of the four was released on Wednesday, while two others -- aged 32 and 49 -- were each charged with two counts of "acts done in preparation for, or planning, a terrorist act", Australian Federal Police said.

They face maximum sentences of life imprisonment if found guilty and are due to appear in a Sydney court on Friday.

The charges came as pilots -- who have to be screened alongside air crew, retail workers and passengers at airports -- said similar requirements were not in place for ground staff, who are instead issued with security cards.

"Pilots and cabin crew are routinely screened along with passengers but a lot of ground staff can access aircraft on the tarmac without the same level of scrutiny," Australian Airline Pilots Association president Murray Butt said late Wednesday.

"We believe it would enhance airport security if all airline staff who have access to aircraft, were screened to the same level as personnel entering through the terminal."

The concerns followed a report by Sydney's Daily Telegraph citing sources who alleged that the plot involved using an unwitting passenger to carry a bomb onboard, with Etihad Airways confirming this week it was helping the investigation.

Aviation experts have also warned of loopholes, such as the use of private-sector security guards instead of government employees at airports, and no photo ID checks for passengers at domestic terminals.

Butt said Australia needed to emulate the US requirement for photo ID checks for passengers, while a former Sydney Airport security chief said security databases should be linked to booking systems.

"The scary thing is domestic airlines have no idea who is really on their aircraft," Mike Carmody told The Australian Financial Review.

"There is very little coordination. Unless you happen to be someone who really stands out, you are going to fly right through security."

-'No room for complacency' 

In response to calls for ID checks, Prime Minister Malcolm Turnbull said Thursday airport security measures were "constantly under review".

He told reporters in Perth that the times passengers had been advised to arrive at airports would return to normal after being extended in response to the alleged plot which "has been disrupted and it has been contained".

Transport Minister Darren Chester defended the current safety measures, saying workers with access to large passenger planes must hold a security card only issued after thorough checks.

"We've endeavoured to toughen up regulations around getting access to those cards, and making sure that people who have access to the airport environment are trusted," he told the Australian Broadcasting Corporation.

"So it's been recognised that there are issues around the world in relation to the threat provided by the so-called trusted insider... and we are taking measures to keep the Australian travelling public safe."

Police have until the weekend to hold the fourth man without charge after obtaining a court extension.

New South Wales Police Commissioner Mick Fuller said earlier Thursday of the allegations against the men: "The evidence gathered is very strong."

Australia's national terror alert level was raised in September 2014 amid concerns over attacks by individuals inspired by organisations such as IS.

A total of 12 attacks, before the latest one, have been prevented in the past few years and 70 people have been charged.

Several terror attacks have taken place in Australia in recent years, including a Sydney cafe siege in 2014 which saw two hostages killed.

source: AFP