Nearly 30,000 "foreign terrorist fighters" are currently in Syria and Iraq, a high-ranking UN official said Tuesday, warning that the risk of attacks in their home countries was growing.

"The number of foreign terrorist fighters is very high" in war-ravaged Syria and neighbouring Iraq, said Jean-Paul Laborde, UN assistant secretary general and head of its Counter-Terrorism Committee.

"There are nearly 30,000, and now that the territory held by Daech (the Islamic State group) is shrinking in Iraq, we are seeing them return, not only to Europe but to all of their countries of origin, like Tunisia, Morocco," he told reporters in Geneva.

"The terrorist attacks in those countries of origin risk getting bigger and bigger to counter-balance the pressure on them" on the ground in Syria and Iraq, he said.

Laborde urged countries to put in place a "filter system to distinguish between the large majority of (returning) foreign fighters, who are not dangerous... and those who are."

The former French judge also stressed that the international community had the judicial tools to fight against terrorism, but warned that "the adaptability and flexibility of terrorist organisations are much faster than ours."

To compensate for the slowness of judicial systems, he called for more cooperation with internet giants like Google, Twitter and Microsoft to help keep tabs on potential terrorists online.

He insisted though that this needed to be done without "violating freedom of expression."

Laborde also called on states to share more information faster.

"If we don't do that, we will continue to see a growing number of terrorist acts," he said.

The Counter-Terrorism Committee, made up of representatives of the UN Security Council member states, was created in New York following the September 11, 2001, attacks in the United States.