Foreign Minister Bob Carr's office has repeatedly tried to confirm the welfare of Muhammad Yusuf al-Kalkuni, who is an Australian-Syrian national, but his fate is unknown more than six months after he was captured.

A list provided to the United Nations General Assembly Security Council in October by the Syrian government names Mr al-Kalkuni among more than 100 foreign nationals who have been arrested for terrorism-related offences. The list has been published online this week.

A spokesman for Mr Carr could not confirm Mr al-Kalkuni's identity. He said the family of an Australian man who they claim was abducted from the city of Dar'a on June 30 had contacted the government asking for assistance.

The list on which Mr al-Kalkuni is named says he was arrested in Dar'a, where it is believed he lived and operated a business. His family is understood to have fled Syria shortly after his arrest, but it is unclear where they are living.

The Syrian government claims Mr al-Kalkuni was involved with a terrorist group in the city, but the nature of his involvement is unclear as the UN translation of that part of the letter says it was "illegible".

Mr Carr's spokesman said the Department of Foreign Affairs officials had attempted to contact the Syrian government through the Hungarian and Egyptian embassies, but had not been able to confirm anything about what the man had been accused of, where he was being held, or whether he was alive.

Three Australians have been killed in Syria, with the families of the men saying they had travelled to the troubled country as aid workers.

It is unclear where Mr al-Kalkuni had lived in Australia, or when he was last settled here, but it is believed he visited as recently as last year.

Two of the Australians killed in Syria were from Melbourne, the other from Sydney.

Free Syrian Army Australian head Zaky Mallah said he encouraged Australians to continue travelling to Syria to provide aid or battle Assad's forces, despite the violence plaguing the country.

Those who travel to Syria to fight could face federal foreign incursion charges, which carry a maximum prison term of 20 years.

Mr Mallah said he feared that Mr al-Kalkuni had been killed and implored the Australian government to save him.

"We need to find out where he is, what's happened to him and why Assad is free to lock up innocent people," Mr Mallah said.

Mr Mallah told Fairfax Media last week that about 250 Australians were providing aid in Syria, and acknowledged that some may be fighting with rebel forces.

Mr Carr's spokesman said there was 67 Australians registered in Syria.

"We have urged all of them to leave as soon as possible," he said.

"It's not safe."