A rapidly increasing flow of Europeans is streaming to Syria to fight on the side of rebel groups aligned with al-Qaeda, raising concerns about homegrown terrorism, top European security officials said Thursday.

The number of Europeans who have traveled to fight in Syria “is estimated at between, more or less, 1,500 and 2,000 people, based on what we’ve heard from our colleagues,” Belgian Interior Minister Joelle Milquet told reporters in Brussels on Thursday, giving an estimate more than double the figure offered last month by U.S. intelligence officials.

E.U. interior ministers gathered Thursday in Brussels to discuss the flow to Syria, which has soared to the top of the counterterrorism agenda as the civil war has entered its third year. Syria has become a gathering point for extremist Islamist fighters from around the world, officials said Thursday. Many fear that combatants could return home and commit acts of terror on European soil or serve as inspiration to others.

European officials said they were stunned by the speed of the radicalization and flow of their citizens to Syria, and some officials urged new measures, such as collecting and sharing more airline passenger information, in an effort to slow the migration.

“Before summer, on the part of France, there were less than 100 French residents who were in Syria. At the moment that I’m speaking, it’s more than 180; 184 to be precise,” French Interior Minister Manuel Valls told reporters in Brussels. And “most individuals have indicated their willingness to fight in organizations close to al-Qaeda,” he added.

Much of the radicalization is taking place online, he said, and in France, 20 percent of those who have gone to Syria have been converts to Islam.

“What we note is the rapidity of the phenomenon,” he said.

Europeans are able to reach Syria far more easily than other past locations of jihadist activity, such as Afghanistan and Iraq. All it takes to get to Syria is a budget flight to Turkey and a quick transit over the porous border. European security officials have said there are an increasing number of training camps on Syrian soil that are composed almost exclusively of Europeans.

The discussions in Brussels this week included Acting Secretary of Homeland Security Rand Beers, as well as interior ministers from Canada and Australia. Dozens of Americans have also either traveled to Syria or attempted to do so but been arrested in the United States, U.S. intelligence officials said last month.

E.U. Counterterrorism Coordinator Gilles de Kerchove met in closed session Thursday with the 28 E.U. interior ministers, where he told them that “the first returnees have come back, and there are cases where individuals continue travelling back and forth,” according to a memorandum signed by de Kerchove that was sent to the ministers ahead of the meeting and was provided to The Washington Post.

The flow of fighters to Syria is a “major security threat,” the memorandum said. De Kerchove urged that law enforcement agencies be given wider access to border data and that the European Union create an independent database of air-travel records — a move that the European Parliament has resisted because of privacy concerns.

Individual European countries have also stepped up their own efforts to stop citizens from fighting in Syria. The Interior Minister of the German state of Hesse, Boris Rhein, this week proposed a nationwide hotline and network of counseling centers to try to combat radicalization. He said a new government-sponsored study in Hesse had found that jihadist recruiters had begun to systematically target students.