By Patricia Zengerle and Mark Hosenball
WASHINGTON (Reuters) - More than 7,000 foreign militants are fighting for the rebels in Syria's civil war and some are being trained to return home and conduct attacks, U.S. spy chiefs told lawmakers on Wednesday.
The estimate, given at a Senate intelligence hearing, was much higher than earlier figures of 3,000 to 4,000 foreign fighters in Syria, and came after news emerged this week that Congress had secretly approved more funding to send weapons to "moderate" rebels.
"We estimate, at this point, an excess of 7,000 foreign fighters have been attracted from some 50 countries, many of them in Europe and the Mideast," James Clapper, the U.S. director of national intelligence, told the hearing.
"And this is of great concern not only to us, but to those countries," he said at the Senate Intelligence Committee's annual hearing on global security threats.
U.S. spy agencies had not previously made the figure of 7,000 public, though it has appeared in classified intelligence reports, a U.S. official said.
Clapper said U.S. intelligence agencies had spotted the appearance of "training complexes" for foreign fighters in Syria, where the war has killed at least 130,000 people and forced millions to flee.
U.S. officials have privately estimated that at any one time, a handful of Americans have been fighting in Syria with rebel factions.
Far larger contingents of foreign fighters have gone to Syria from other countries. European authorities have estimated that at any one time, as many as 100 British men are training and fighting with Syrian rebels, often with the most militant factions.
U.S. and European officials have expressed concern that because there are so many land routes into Syria from other countries, it is much harder for security agencies to track the travel arrangements of would-be foreign fighters than it is for would-be militants heading to more remote battlegrounds such as Pakistan, Yemen or North Africa.
U.S. and European security officials said this week that light arms supplied to the United States are flowing to moderate Syrian factions and the U.S. Congress had secretly approved funding for months of further deliveries.
Congressional committees had held up weapons deliveries for months over fears that U.S. arms would not prove decisive in the rebels' efforts to oust Syrian President Bashar al-Assad and his government and could end up in the hands of Islamist militants.
Some of the militants fighting to oust Assad, such as the al-Nusra Front, have aspirations to attack the United States itself, Clapper said.
Senator Dianne Feinstein, who chairs the intelligence committee, called the situation in Syria the most notable new security threat in the year since the panel last held a hearing on global threats.
(Editing by Alistair Bell and Tom Brown)