Lukas Dam was reportedly killed in a US airstrike on Kobane in December 2014.

Would-be Kiwi Jihadis have been "talked down" by the Security Intelligence Service after their worried families got in touch, Prime Minister John Key says.

A New Zealand mother is grieving the death of her son after he died fighting for the Islamic State in Syria.

Karolina Dam, formerly of Auckland but now living in Copenhagen, Denmark, told Radio New Zealand her Danish-born son, Lukas, left for Syria in the middle of 2014 and was killed by an American airstrike in December.

politiken.tv Karolina Dam said her son Lukas left for Syria in the middle of 2014 and was killed by an American airstrike in December.

That was the account given to her by four of Lukas' fellow Islamists via Facebook message, who celebrated the death of the 18-year-old as "martyrdom" in Kobane, RNZ reported.

Key could not say if Dan had been in touch with New Zealand authorities about her son but said it was "not unheard of" for families to raise concerns about radicalised relatives.

"There are some people that we believe we've actually talked down from wanting to get engaged and leave," Key said Monday.

Asked who had talked to them, Key confirmed it was the SIS.

He would not give numbers but confirmed there were still a handful of Kiwis believed to have joined IS in Iraq and Syria.

Dam told Danish news site Politiken her son had a troubled childhood because he suffered from autism and attention deficit disorder.

But at the age of 15, he met an Islamic group, converted to Islam and had seemed to turn his life around.

He was no longer committing crimes and helped raise funds for the victims of Middle East warzones but at the same time he was being radicalised by an extremist, Dam said.

Her son originally travelled to Syria as a "relief worker" and spent his first few months around the cities of Atmeh and Idlib where his mother was able to keep in contact with him.

But he later joined the Islamic State and was moved to the rebel movement's self-declared caliphate of Kobane.

Dam told RNZ she had no definitive evidence that her son had been killed but had four accounts of how he died guarding a building.

"I need to believe (he is dead)," she said.

"I need peace and quiet and I need to get on. I don't know if he's alive. I don't know if he's in jail. I don't know if Islamic State have killed him."

She also blamed the Copenhagen City Council for not doing more to help her son.

The council was responsible for integration and anti-radicalisation programmes.

It had taken Lukas into its care when he was a troubled teenager but had not put him in either of the programmes.

The council would not comment to RNZ but Dam said she received an apology from Copenhagen's deputy mayor Anna Mee Allerslev.

Dam said she told them of her son's radicalisation but the anti-radicalisation programme was not given a report on Lukas until four months after he left.

Dam said she was happy with the apology.

"I can't do anything about it now, except to try to prevent it from happening again. And that would be in the spirit of my son. That's what I need to do."

"I curse the hell out of Islamic State", she said.

Lukas Dam was believed to be the 19th Danish national to die in Syria out of an estimated 115 people who have travelled there to participate in military battles, Politiken reported.