TERROR ALERT HIGH IN AUSTRALIA.

Australia has raised its terror alert level to "high" for the first time since the national alert system was introduced in 2003.

According to the four-level system, a reading of "high" means a terrorist attack "is likely," however the country's Prime Minister Tony Abbott said no specific threat had been made

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"This does not mean a terror attack is imminent, we have no specific intelligence of particular plots. What we do have is intelligence that there are people with the intent and capability to mount attacks," Abbott said at a press conference in Canberra on Friday.

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What does "high" mean?

Australia introduced its alert system in 2003, with four levels of risk: low, medium, high and extreme. It's been at "medium" since the system began, and is defined as meaning a "terrorist attack could occur."

The elevation to "high" would not make much difference to daily life for most people, the prime minister said.

"What people would probably notice though is more security at airports, more security at ports, more security at military bases, more security at government buildings and more security at large public events," he said.

Why now?

The decision to raise the level was foreshadowed by the departing director-general of the Australian Security Intelligence Organization (ASIO) earlier this week.

David Irvine said the threat level had been building in Australia over the last year, due to the increasing influence of jihadists fighting for Islamic extremists ISIS in the Middle East who had recruited Australians to fight.

"They are of concern because, if they come home, they come home with training and with potentially increased intent," he told the ABC.

Officials believe there are at least 60 Australians fighting with terror group ISIS, and as many as 100 "facilitators" still in the country.

A number of arrest warrants have been issued for suspected terrorists fighting abroad, and police have swooped on a number of suspects in Australia, most recently arresting two men in Brisbane for allegedly recruiting and funding fighters sent to Syria.

Alert levels elsewhere

Australia's higher alert level follows an increase in the United Kingdom, which raised its level from "substantial" to "severe" in late August.

Again, authorities said there was no intelligence to suggest an attack was imminent. However, hundreds of British citizens are believed to have joined jihadists abroad. Notably, a hooded man with a British accent appeared to be the executioner of U.S. journalists James Foley and Steven Sotloff on videos released by ISIS.

The U.S. replaced its color-coded warning system with the National Terrorism Advisory System (NTAS) in 2011. NTAS alerts are only issued if there's "credible information about a threat." And, alerts are classified as either "elevated" or "imminent."

France abandoned its color-coded alerts earlier this year, and instead now relies on a two-level "Vigipirate" system. When more vigilance is needed, the triangle logo is displayed in public places. If there's specific intelligence of an attack, the logo carries the words "alerte attentat" or "attack alert."

CNN.