Christopher Voss, a 24-year FBI veteran, says the 12-hour mark of a siege is an important mark for a kidnapping situation involving just one gunman.

Police have identified Sheik Man Haron Monis, a self-styled Islamic State preacher out on bail for accessory to murder, as the gunman who is holding 15 terrified hostages in the cafe.

Gunman ... Iranian-born, self-styled Islamic State cleric Sheik Man Haron Monis chained himself to a railing after appearing at Downing Centre Court in Sydney for sending offensive letters to families of Australian soldiers killed in Afghanistan. Picture: News Corp Australia Source: News Limited

“He’s going to be running out of gas before long,” Mr Voss told CNN on Monday.

“Usually in these types of sieges, a person by themselves, at about the 12-hour mark they become psychologically overwhelmed by trying to manage the entirety of this siege.”

The siege is the top news story in the US, with President Barack Obama briefed on the situation by his top counterterrorism adviser, Lisa Monaco.

WORLD REACTS TO SYDNEY SIEGE

US authorities, including the New York Police Department, are monitoring the Sydney situation and are on alert for any copycat responses in US cities.

Terrified ... A hostage runs to police after escaping the siege in the Lindt Cafe in Sydney's Martin Place. Photos: Chris McKeen Source: News Corp Australia

Australian law enforcement has received high praise for their handling of the siege.

Former White House security adviser, Fran Townsend, described the Australian intelligence community as very capable.

“This is a textbook operation for tactical unit, on scene commanders, negotiators,” retired Bureau of Alcohol, Tobacco, Firearms and Explosives special agent hostage negotiator Jim Cavanaugh told MSNBC.

“This is the way they operate.

“From everything I have seen they are doing it according to the book.

“Australian police have great tactical units and negotiators so they know what to do and are doing the right things.”