SYDNEY, July 30 -- Security has been tightened at Australian airports after police foiled "Islamic-inspired" plans for a bomb attack on an aircraft during counter-terrorism raids in which four men were arrested, the Australian Federal Police (AFP) said on Sunday.

The increased security presence caused minor delays for some air transport operators on Sunday and authorities warned travellers to prepare for potentially more significant disruptions as traffic picked up during the week.

"In recent days, law enforcement has become aware of information that suggested some people in Sydney were planning to commit a terrorist act using an improvised device," AFP commissioner Andrew Colvin said during a press conference with Prime Minister Malcolm Turnbull on Sunday.

"We do believe it is Islamic-inspired terrorism. Exactly what is behind this is something that we will need to investigate fully.

"At this time we don’t have a great deal of information on the specific attack, the location, date or time. However, we are investigating information indicating that the aviation industry was potentially a target."

Five properties were searched on Saturday across the Sydney suburbs of Surry Hills, Lakemba, Punchbowl and Wiley Park. The commissioner said four of those searches may continue for days.

An AFP spokesman told Reuters the four men, who were arrested on Saturday, had not been charged as of Sunday afternoon.

Turnbull said advice from Australian security and intelligence agencies had led to increased security measures at Sydney airport on Thursday with tighter security implemented at the country's other domestic and international airports on Saturday.

"Some of the measures will be obvious to the public, some will not be," Turnbull said.

Colvin said travellers could expect an increased police and security agency presence at airports.

"You can expect longer delays to make sure that more screening is being done on baggage, both hold luggage as well as hand luggage," adding that travelers should allow more time to get through security.

Airlines Qantas Airways and Virgin Australia notified passengers via text messages, social media and emails to expect delays though disruptions at the nation's airports appeared to be minimal on Sunday.

A spokesman for Adelaide Airport reported delays of about half an hour for some flights but said traffic was otherwise smooth. A spokesman for Melbourne Airport, Australia's second busiest, said it was preparing for delays during peak weekday periods.

Australia, a close ally of the United States, has been on heightened alert for attacks by home-grown militants returning from fighting in the Middle East, or their supporters, since 2014.

Authorities say they have thwarted a number of potential attacks since then but there have been several "lone wolf" assaults, including a cafe siege in Sydney that left two hostages and the gunman dead.

About 100 people have left Australia for Syria to fight alongside organizations such as Islamic State, Australia's immigration minister said last month.