FILE PHOTO: Jean-Paul Laborde, executive director of the U.N. Counter-Terrorism Executive Directorate (CTED), gestures during a news conference at the end of a CTED special meeting at El Pardo Palace, outside Madrid, Spain, July 28, 2015. REUTERS/Sergio Perez

BRUSSELS (Reuters) - Europe will face an exodus of Islamic State fighters defeated in Syria and Iraq this year, more dangerous and battle hardened than previous returnees, the head of the U.N. Security Council’s counterterrorism agency said on Thursday.

Speaking to reporters following meetings with EU officials, Jean-Paul Laborde said several European countries estimate that the rates of fighters returning from conflict zones has increased by one-third over the past year.

Threat levels in Western Europe are high following a spate of deadly attacks that killed dozens over the past two years, with many nations concerned about their ability to guard their borders.

Laborde said the foreign fighters seeking to return to Europe now are much “more dangerous” than previous waves of returnees, full of resentment after years on the battlefield including in recent confrontations.

Islamic State has lost large swathes of territory during military campaigns, including by the Russian-backed Syrian army and U.S.-backed militias.

“On average, these people are much more committed, more experienced and more skilled,” Laborde told reporters.

“In spite of the travel restrictions ... still you will have a number of foreign terrorist fighters which will probably slip through the borders and go back, come back to these countries, especially with smuggling networks,” he said.

However, restrictions have worked to help dramatically curb the flow of foreign terrorist fighters leaving to join the ranks of the Islamic State, he said.

Based on estimates compiled from governments, the U.N. says 40 to 50 percent of some 30,000 foreign fighters, not all of them from Europe, have already left territories controlled by the Islamic State group.