A top U.S. counterterrorism official has warned that the number of foreign fighters travelling to Iraq and Syria to join Islamic State (IS) militants group is increasing at an alarming rate.

Nicholas Rasmussen, director of the National Counterterrorism Center said that more than 20,000 fighters have travelled from more than 90 countries to join IS.

Rasmussen said that the number of fighters travelling to Syria is “unprecedented”.

He added that it exceeded the number of travellers who went to Afghanistan and Pakistan, Iraq, Yemen or Somalia at any point in the last 20 years.

Officials said that around 3,400 fighters are believed to have come from western countries wherein 150 are from the United States.