Nearly 30 000 people, including more than 250 Americans, have left their countries to join militant Islamists fighting in Syria and Iraq, many with the Islamic State (ISIS), according to a congressional study of the issue released on Tuesday.
The six-month study by Republicans and Democrats on the House of Representatives Homeland Security Committee said the US government lacks a strategy for combating such travel, and called for measures including better information sharing, within the United states and internationally.
Thirty thousand doubles the total estimated number of such fighters a year ago, when authorities also said about 100 Americans had joined, or tried to join the fighting in Syria and Iraq.
Several dozen of the fighters have made their way back to the United States, the report said.
"We are witnessing the largest global convergence of jihadists in history," the report said.
The report found that authorities have failed to interdict the majority of Americans who have traveled to join foreign conflicts.
It also found there is too little assistance providing local communities with the ability to spot warning signs.
The study also criticized foreign governments, particularly in Europe, for failing to take measures such as screening travellers against terrorism watch lists or trying to identify forged passports.