WASHINGTON - Some 12,000 foreign jihadists from 50 countries, including Americans, have gone to fight in Syria since the conflict began, the US State Department said Thursday.
A US official, speaking on condition of anonymity, estimated that more than 100 US citizens have travelled or tried to travel to Syria to join the conflict.
They have travelled to join radical groups including the Islamic State (IS), militants fighting the regime of Bashar al-Assad in Syria who have expanded into neighbouring Iraq.
"We think that there are approximately 12,000 fighters from at least 50 countries in Syria - foreign fighters, including a small number of Americans - that may have travelled to Syria since the beginning of the conflict" more than three years ago, said deputy State Department spokeswoman Marie Harf.
"They may all not still be there," she said, without revealing an estimated number of Americans.
US officials have been unable to identify an organised recruitment effort aimed at US citizens, as is the case in Europe.
Washington has publicly stated that Western fighters pose an especially dangerous risk, especially when they return to their countries of origin.
In late September, President Barack Obama is scheduled to host a Security Council summit with other heads of state focusing on the acute threat posed by foreign fighters in Syria and Iraq.
During his August 12 visit to Sydney, Secretary of State John Kerry said that Washington and Australia agreed to take concerns about the threat posed by jihadist foreign fighters in Syria, Iraq and elsewhere to the United Nations.