Australian counter-terrorism police said on Wednesday they arrested two men in Sydney, eight days after a 16-hour siege in a central city cafe ended with the deaths of two hostages and a gunman with radical Islamist sympathies.

Australian Prime Minister Tony Abbott said on Tuesday that security officials had intercepted a heightened level of "terrorist chatter" in the aftermath of the Sydney cafe siege, but there were no specific threats of attacks.

A 20-year-old man was charged with being in possession of documents designed to facilitate a terrorist attack and a 21-year-old was charged with breaching a control order, police said.

The documents had mentioned potential government targets but were not directed at the prime minister, Australian Federal Police Deputy Commissioner Michael Phelan told a media conference in Sydney.

Australia, a staunch ally of the United States and its action against the Islamic State inSyria and Iraq, is on high alert for attacks by sympathizers of the radical group and from home-grown militants returning from fighting in the Middle East.

Police said they had now arrested and charged 11 people with terrorism-related offences since launching massive raids in Sydney and Brisbane in September, soon after raising the terror threat to "high" for the first time.

The most recent arrests came after the monitoring of a group of 15 to 20 people who police believed were actively supporting Islamic State through funding, sending fighters and planning attacks.

"Certainly their ideology is linked to Islamic State overseas," Phelan said.

Man Haron Monis, a self-styled sheikh who was facing numerous charges for violent crimes, held hostages at gunpoint at the Lindt Chocolate Cafe in Martin Place, a centralSydney shopping and office precinct, from mid-morning on Monday last week.

Two hostages, cafe manager Tori Johnson and lawyer Katrina Dawson, were killed along with Monis when police stormed the cafe. An official investigation into the final moments of the siege and the deaths of all three is underway.