U.N. experts say increasing numbers of foreign fighters for the Islamic State group and al-Qaida are returning home, potentially to carry out attacks, and are using the "dark web" or encrypted messaging that the most sophisticated intelligence agencies can't penetrate.

The experts monitoring sanctions against the extremist groups say in a report circulated late Wednesday that governments highlighted the challenge to national security from the communication methods being used by these "foreign terrorist fighters" and people being radicalized at home who need to be monitored and investigated.

The experts said the rise in the use of the "dark web" and encrypted messaging "has closed off the ability of even the most sophisticated agencies to penetrate huge quantities of messages."