More than 25,000 foreign fighters from 100 nations have travelled to join militant groups such as al-Qaeda and Islamic State (IS), a UN report says.
It said the number of foreign fighters worldwide had soared by 71% between the middle of 2014 and March 2015.
Syria and Iraq were by far the biggest destinations and had become a "finishing school for extremists".
It also said if IS were defeated in Syria and Iraq, the foreign fighters could be scattered across the world.
The UN Security Council had asked experts six months ago to investigate the threat from foreign fighters joining Islamic State and other militant groups.
In the report filed to the council late last month, the experts say the flow of foreign fighters has risen from a few thousand a decade ago and is now "higher than it has ever been historically".
They say: "For the thousands of [foreign fighters] who travelled to the Syrian Arab Republic and Iraq... they live and work in a veritable 'international finishing school' for extremists, as was the case in Afghanistan during the 1990s."
Syria and Iraq were said to house 22,000 foreign fighters, with 6,500 in Afghanistan and hundreds in Yemen, Libya, Pakistan and Somalia.
Foreign fighter case studies
Texan Michael Wolfe, 23, planned to fly to Turkey via Iceland and then get into Syria to commit "violent jihad". But he unknowingly relied on an undercover FBI agent for travel advice, was arrested and admitted attempting to provide material support to terrorists.
Melbourne schoolboy Jake Bilardi travelled to fight with IS and reportedly died in a suicide attack in Iraq this year. The 18-year-old Muslim convert left Australia last year and flew via Turkey to Iraq.
Briton David Souaan was jailed in December 2014 for preparing terrorist acts. Arrested at Heathrow last May after a tip-off from fellow students. Had previously been in Syria and had pictures of himself posing with guns.
A high number of foreign fighters had come from Tunisia, Morocco, France and Russia but there has also been an increase from the Maldives, Finland and Trinidad and Tobago.
The report called for greater intelligence sharing between nations to help identify foreign fighters.
It highlighted the effect of social media networks which had linked "diverse foreign fighters from different communities across the globe".
The chances of foreign travellers becoming caught up in terrorist incidents was "growing, particularly with attacks targeting hotels, public spaces and venues".
The Security Council adopted a resolution in September demanding all states make it a serious criminal offence for their citizens to travel abroad to fight with militants, or to recruit and fund others to do so.