(Transcript from World News Radio)

Australia and other countries are growing increasingly concerned about the involvement of their citizens in the seemingly intractable civil war in Syria.

The main concern is that those who get involved may carry out acts of violence on their return home.

Measures to try to limit the problem, already implemented or proposed by some countries, include cancellation of citizenship.

Appeals are also being made to Muslim leaders - and the families of potential fighters - to try to limit foreign involvement in the Syrian war.

Greg Dyett has the details.

In a recent speech in Washington, the federal Attorney General George Brandis made it clear Australia is contributing to a problem that has many countries worried.

"I'm sorry to have to tell you that per capita, Australia is one of the largest sources of foreign war fighters to the Syrian conflict from among countries outside the immediate vicinity."

The fear is the fighters, once radicalised in Syria, will return to their countries and pose a terrorist threat.

In Washington, the United States Secretary of Homeland Security Jeh Johnson told America's ABC Network the US is on alert for the same problem.

"Syria has become a matter of homeland security. We're very concerned about Syria foreign fighters, people who are going into Syria, being recruited by extremists there."

A number of Australians have already been killed while fighting with anti-government rebel forces in Syria.

George Brandis says the Syrian conflict is drawing Australians on a much bigger scale than other conflicts.

"The number of Australians participating in the conflict in Syria is higher than we've experienced in previous conflicts with assessments of between 120 and 150 Australians travelling to the greater Syria region to participate in the conflict."

Last December, the Australian Security Intelligence Organisation cancelled the passports of 20 men in Western Sydney, following reports that they could engage in politically-motivated violence if they were allowed to travel overseas.

ASIO has also cancelled passports to prevent travel by other people, for the same reasons.

Reports in Britain suggest up to 20 men have died in the Syrian conflict and that's resulted in counter-terrorism police making an unprecedented appeal to Muslim women to try to persuade their relatives not to go to fight there.

Deputy Assistant Police Commissioner Helen Ball told the BBC part of the campaign involves encouraging people who are concerned to phone a police hotline.

She says the police have managed to convince some people not to travel to Syria.

"Our experience is that people are very positive about this. They do want to talk about their concerns. They want to talk about how people are suffering in Syria and what they can do to help. And we want to help them to have alternative ways of helping the people of Syria rather than them going into fighting themselves."

The President of the Australian Syrian Association of Victoria, Said Al Jouni, says it's been warning everyone not to travel to Syria, including people who say they want to do humanitarian work.

"To spread the message to those people because those youth they don't know the full picture about exactly what will happen when you end up even in Syria or in Turkey. The message should be past to them that you will not be on your own, you cannot control yourself, you are under the command of other people."

Mr Al Jouni says imams should be doing more to get that same message across in Australia's mosques.

"Imams could be very valuable tool to spread such message especially when we talk about those people who are like not deeply thinking about their situation. So imams would have significant influence on those people so they could be good collaborator also in this process."

The Australian National Imams Council says such efforts have already been underway for some time in conjunction with Australian law enforcement agencies.

Sheikh Mohamadu Saleem is from the council's Victorian chapter.

He says in part, it's motivated by the law that makes it illegal for Australians to take part in any anti-government rebel military action overseas.

"The program is called Generation Change, which was designed to engage the youth in a positive way to respond to the Syria conflict. And the Imams Council supported that because we don't want people to be caught in an unnecessary legal battle by engaging in illegal activities overseas."

Sheikh Saleem says there's room for the imams to do more, including reaching out to those families who have lost loved ones in the Syrian bloodshed.

"This losses definitely will affect the Muslim community here originally from Syria and to help them to overcome these emotional issues, to provide them with counselling or assistance in such a way to reduce the impact of this emotional experience."

Discussions are underway in Europe on new domestic and international measures that could be taken to deal with the issue.