OTTAWA (AFP) - Canada is concerned about the national security threat posed by citizens who joined the Islamic State in Iraq and Syria (ISIS) group, returning to this country, Prime Minister Justin Trudeau said on Monday.

In Parliament, Trudeau pledged to prosecute those who broke Canada's anti-terrorism laws by joining the ISIS group, but also said his government would try to reintegrate them into society.

"We recognise the return of even one individual (who joined the ISIS group) may have serious national security implications," Trudeau said.

"We are going to monitor them. We are also there to help them to let go of that terrorist ideology," he added.

Nearly 180 Canadians are known to have travelled overseas to join the ISIS group. About 60 have returned to Canada, according to government figures released in 2016.

Only two returnees have been prosecuted under Canada's anti-terrorism act, so far.

Public Safety Minister Ralph Goodale last week explained the difficulties of gathering evidence in war zones for successful criminal prosecutions.

He rejected, however, the use of extrajudicial killings used by allies to solve the problem, telling public broadcaster CBC: "Canada does not engage in death squads."