The head of the U.N. Security Council's counterterrorism agency says the Islamic State group is proving more flexible and adaptable than the governments battling the militants.

Speaking to reporters after addressing diplomats in Geneva, Jean-Paul Laborde said governments need to coordinate and share information more, and acknowledged that his job was partly to help them do so.

Based on information compiled from governments, he estimated at around 30,000 the number of foreign fighters in areas of Syria and Iraq now controlled by the Islamic State group.

He said the radical group has shown a "remarkable ability to adapt" such as seeking new funding sources like drug smuggling, even as the territory it controls has been shrinking due to international military pressure.

Laborde says: "We have to be more flexible."