SYDNEY (Reuters/AFP) - The Australian authorities have detained seven young nationals as they were attempting to fly to the Middle East, Prime Minister Tony Abbott said on Thursday, over suspicions they wanted to join Islamist militant groups.

"We have stopped at the airport, seven young Australians who were planning to travel to the Middle East it seems to join terrorist groups over there," Mr Abbott told reporters in Canberra.

The Sydney Daily Telegraph said five of the men, aged in their 20s and 30s, all tried to leave through Sydney Airport together, on a flight initially to Malaysia, on Aug 12, triggering an alert due to the sums of money they were carrying.

Each of the five were found to be carrying about A$10,000 (S$10,300) when their luggage was searched, Australian Border Protection Minister Peter Dutton told reporters.

Another two men were stopped when they attempted to board a flight from Sydney over the weekend. Mr Dutton said those two had connections with the five men detained earlier.

Citing an intelligence source, the report said some of the suspects were known to the authorities and their passports were immediately suspended under new laws that came into effect this year.

Mr Abbott would not confirm the details, calling it an "operational matter".

He praised the border force and counter-terrorism units at airports for doing their jobs in this "particularly important and significant way".

Australia is on high alert for attacks by radicalised Muslims, including home-grown militants returning from fighting in the Middle East, having raised its threat level to "high" and unleashed a series of high-profile raids in cities.

In addition to attempts to safeguard against any domestic attacks, Australian officials have intensified efforts to prevent radicalised citizens from joining organisations such as Islamic State in Iraq and Syria.

"We are concerned about the number of people presenting at airports, particularly younger people, who might be seeking to travel overseas for reasons that would horrify Australians and their parents and family and community no doubt as well," said Australian border Protection Minister Peter Dutton said on Thursday.

Mr Abbott has told Parliament at least 70 Australians were fighting in Iraq and Syria, backed by about 100 Australia-based "facilitators".