OTTAWA -- The number of Canadians heading overseas to fight in Syria and Iraq appears to have levelled off, Canada's spy agency says.

The Canadian Security Intelligence Service is not seeing the same kind of jumps in the numbers of terrorist travellers that it saw two years ago, says Brian Rumig, the service's assistant director for operations.

"Those numbers have settled down," Rumig said during an appearance Monday at the Senate's national security and defence committee. "They have levelled off."

However, Rumig cautioned the situation is fluid and figures fluctuate monthly.

CSIS is aware of 180 individuals with connections to Canada who are suspected of terrorist activity abroad. Half of these individuals are believed to be in Syria and Iraq.

The spy service also knows of approximately 60 such people who have returned to Canada.

The figures are consistent with ones the spy service included in an August report as well as those made public in February by CSIS director Michel Coulombe.

Terrorism, including terrorist travel and the radicalization of Canadians, remains the most prominent threat to Canadian security, Rumig said. "The intelligence community has never before faced a terrorist threat of this scope, scale and complexity."

There are more than 30,000 foreign fighters in Syria and Iraq, where the Islamic State of Iraq and the Levant, also known as Daesh, has rampaged through huge swaths of territory, Rumig said. The group continues to face pressure from forces struggling to liberate occupied areas.

The rate at which such fighters have left the conflict zone has "declined steadily" during the last two years -- likely as a result of ISIL's increased control and influence over these people as well as the ability of regional authorities to better control their borders, Rumig added.

"Daesh's continued loss of territory in Syria and Iraq, however, will potentially reverse this trend, driving more fighters to leave, possibly returning to their home countries."

Many may never return home, he said. And despite CSIS's best estimates, "the number of Canadians actually returning is an unknown."

Many are so committed to the cause they have no intention of coming back to Canada, Rumig said. They will continue to fight with ISIL even if it reconstitutes itself in another part of the Middle East or elsewhere.

Even so, CSIS is working closely with other agencies to try to assess the threat returnees might pose, he said.

In some cases, initial suspicions that someone might have participated in terrorist activities give way to the belief that they are not in fact an extremist.