The men detained in counter-terrorism raids on the weekend may have already made an attempt to smuggle their homemade bomb onto an international flight prior to raids, law enforcement officials have told 7.30.

Key points: Foreign tip-off led to police uncovering plot to bring bomb onboard flight in meat grinder

Foreign tip-off led to police uncovering plot to bring bomb onboard flight in meat grinder Intelligence agency intercepted messages from conspirators in Syria

Intelligence agency intercepted messages from conspirators in Syria Raids fast-tracked after British Government warned they could issue Australian travel warning

Raids fast-tracked after British Government warned they could issue Australian travel warning Arrested men described as an "organised cell" with "technical capabilities" and access to materials that "pose a credible risk"

The men may have then developed an alternate plan after that failed attempt and tried to get the device onto a domestic flight.

"We have a quite sophisticated terrorist conspiracy, in contrast to several of the more low-tech unsophisticated conspiracies which have been thwarted in Australia in recent times," security expert Neil Fergus said.

Behind the plot are a small group of Sydney men with deep connections to Islamic State (IS).

The apparent instigator of the plot is Khaled Khayat, whose brother is believed to be a senior fighter with IS in Syria.

Khaled Khayat's relative, Mahmoud Kayat, is also being held by police.

Khaled Khayat (L) and Mahmoud Khayat (R) were arrested in the counter-terrorism raids. ( Supplied )

The other two men arrested, Abdul El Karim Merhi and Khaled Merhi, are related to Ahmed Merhi, who travelled to Syria in 2014 and is fighting for IS.

"These networks, whether they're familial or otherwise, are very important," said Jacinta Carroll, head of the Australian Strategic Policy Institute's counter-terrorism policy.

"That's why Australia and other countries have been so concerned about preventing the travel of their own individuals as foreign fighters."

In a Facebook post, Ahmed Mehri called on Muslims to undertake suicide attacks, known as istishhadi.

"You must continue with istishhadi operations for, by Allah, it is very effective poison against the enemy's [sic] of Allah."

Sorry, this video has expired Five properties were raided on Saturday night

Tip-off came from overseas

The conspiracy to smuggle the bomb onto a flight, hidden in a meat grinder, was only uncovered when a foreign intelligence agency intercepted communications to the conspirators from Syria.

"They have made this disruption only three days after hearing a tip-off, presumably from a partner agency overseas, that attack-planning by this cell was imminent," Ms Carroll said.

7.30 has learned police hoped to gather more evidence before arresting the men, but the British Government declared they would issue a public security alert for Australian travel if the raids did not happen promptly.

Travellers experienced heightened security measures at Australian airports after the raids. ( ABC News: Ewan Gilbert )

Ms Carroll said British concern may have been so high because of the level of sophistication the accused terrorist cell exhibited.

"What we've seen with this attack is that there's an organised cell that has a level of technical capabilities and access to materials that authorities have assessed pose a credible risk," she said.

"And unusually, in light of the other plots we've had in Australia, this is one that was seeking to attack a very hard target."

Mr Fergus said the well-made bomb and connections to IS may be bad news for global security.

"If this does, as it looks like it will, turn out to be an IS-inspired or -directed plot, and if the technology which is being used is of a similar level to what we've seen from Al Qaeda, then the threat globally has just risen a notch, there's no question about that."