The estimate, given at a Senate intelligence hearing on Wednesday, was much higher than earlier figures of 3,000 to 4,000 foreign terrorists in Syria, and came after news emerged this week that Congress had secretly approved more funding to send weapons to militants in Syria.
"We estimate, at this point, an excess of 7,000 foreign terrorists have been attracted from some 50 countries, many of them in Europe and the Mideast," James Clapper, the US 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.
US spy agencies had not previously made the figure of 7,000 public, though it has appeared in classified intelligence reports, a US official said.
Clapper said US intelligence agencies had spotted the appearance of "training complexes" for foreign terrorists in Syria, where the unrest has killed at least 100,000 people and forced millions to flee.
US officials have privately estimated that at any one time, a handful of Americans have been fighting in Syria with militant factions.
US and European security officials said this week that light arms supplied to the United States are flowing to Syrian militants and the US Congress had secretly approved funding for months of further deliveries.
Congressional committees had held up weapons deliveries for months over fears that US arms would not prove decisive in the militants' efforts to oust the Syrian President government and could end up in the hands of extremist militants.
Some of the foreign terrorists fighting in Syria, 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.
IHS Jane's Company estimated that some 10,000 militants are fighting for groups affiliated with al-Qaeda such as al-Nusra Front, and the rest fight for different militant groups.
NTJ/NJF