A fighter from the Free Syrian Army rebel group runs to avoid sniper fire from forces fighting for President Bashar al-Assad in the northern Syrian city of Aleppo on Tuesday. Reuters

Scores of jihadist fighters from Europe who streamed to Syria to join Islamic extremist rebels have begun returning home, where some are suspected of plotting terror attacks, according to U.S. and European intelligence and security officials.

Authorities in the U.K. and France recently made several terror-related arrests of individuals suspected of links to Syria.

"They're real committed jihadists," a senior U.S. intelligence official said. "The concern is that we're at the very early stages of this."

For the U.S. and Western countries, the returning jihadists pose the biggest long-term concern of the Syrian civil war, the official said. Governments are rushing to counter the new terror threat.

"We monitor very closely people seeking to travel [to Syria]—and also people traveling back—because of the potential risk they may pose upon their return to the U.K.," said Britain's security minister James Brokenshire.

The total number of fighters from Europe is difficult to track, but officials and academics estimate it at about 1,000 or more, including from Germany, France and the Netherlands. Dozens have traveled to Syria from the U.S.

Once there, many are believed to fight alongside al Qaeda-affiliated groups such as Jabhat al Nusra, or the Nusra Front, and the Islamic State of Iraq and al-Sham, known as ISIS.

The European Union doesn't ban membership in Syrian groups affiliated with al Qaeda, which makes it difficult to crack down on the flow of jihadists to the war.

The U.S. has designated ISIS and the Nusra Front as terrorist groups and countries such as the U.K. are pressing to do the same.

The flow of fighters to causes in the Middle East started with Afghanistan in the 1980s and continued during the Iraq war.

But the number going to Syria has mounted more rapidly, U.S. and European officials said.

Recruited through a network of mosques across Europe, these jihadists then make the pilgrimage to safe houses in southern Turkey, where they prepare to cross the border into Syria's battlefields.

The recruiting efforts in Europe's mosques aim for Muslim youth with clean records who aren't on the radar of intelligence services. This makes it easier for them to return home later, the European diplomat said.

An international Islamic group, Hizb al Tahrir, is at the center of this recruitment in Europe, Western officials say. The group is particularly strong in the U.K. and Denmark, the European diplomat said.

"They create small groups and form a strong sense of group cohesion with a leader in the middle…surrounded by young, aspiring jihadists," the European diplomat said.

They also show videos and photos of the war's human toll for emotional appeal.

European governments are most concerned about ISIS fighters returning because that group "wants to use Syria as an al Qaeda operations headquarters for global terror," this diplomat said.

At ISIS and Nusra Front safe houses across southern Turkey, fresh recruits from Europe, Australia and to a lesser extent, the U.S., turn in their Western passports and receive a Syrian I.D. They are issued a nom du guerre and cross the border to Syria's battlefields.

"The procedure for Jabhat and ISIS is to hold the passports centrally," the European diplomat said. "Those passports can be reused, and they can now go anywhere in Europe."

Western officials believe members of these groups may also be faking their deaths so their biometric data is wiped clean from European databases and they can re-enter Europe undetected.

ISIS takes these European passports and distributes them to other jihadists who look similar, allowing them to enter Europe for operations.

For now, the most dedicated jihadists remain in Syria. But U.S. and European officials have said scores have filtered back and expect that as the conflict drags on, more will.

Security officials are concerned because, once inside Europe, the returning fighters can move across borders with relative ease.

Belgium is just a two-day drive from Syria and from there, an undetected jihadi with a European passport could make his way to the U.S. virtually unimpeded.

The jihadists "are a serious concern to our countries because they can and will return battle-hardened, further radicalized, traumatized" and more closely connected with extremist groups, Dick Schoof, national coordinator for security and counterterrorism in the Netherlands, said at a recent security conference in London.

British police in September arrested two brothers, both U.K. citizens in their 20s, in relation to alleged terror training in Syria.

In mid-October, several men were arrested in London on suspicion of involvement in a terrorist plot.

Authorities have been investigating whether there was a Syria connection.

French police in November placed under formal investigation four men who were allegedly in touch with Islamist groups within Syria and were organizing trips of jihadist candidates from France. The men, all French nationals between 22 and 35, are in custody but haven't been charged.

Concern about the issue has prompted Dutch authorities to raise the terror threat level in the Netherlands and increase resources for monitoring jihadist travel.

And with fighters still flocking to extremist groups in Syria, security officials are trying to find ways to stem the flow and keep track of those who go.

U.S. intelligence officials, working with European counterparts, have redoubled efforts to monitor jihadist recruitment networks in Europe and the West, focusing on those returning home.

That means identifying individuals, tracking the facilitation routes the recruitment networks use, and following financial transfers.

"We assume we're way undercounting," the senior U.S. intelligence official said.

European and other governments often can do little to prevent someone from traveling to Syria.

But in April, the U.K. announced new rules making it easier for British authorities to confiscate the passports of Britons suspected of traveling to training camps or other terrorist-related activity.

In the Netherlands, authorities recently clarified laws designed to prosecute individuals traveling to or returning from jihadist conflict zones and in October, secured their first conviction under the newly amended laws.

France last year introduced a law to allow authorities to prosecute people planning terror attacks abroad and those suspected of alleged terrorist activity overseas.

Police in France are also following French nationals who return from Syria to try to identify jihadists, an official at the French antiterrorism prosecutor's office said. About 20 of them, many with dual Middle Eastern or North African citizenship, are under formal investigation.

—Inti Landauro in Paris contributed to this article.

Bound for Syria's Battlefields

About 1,000 Europeans have joined the war in Syria

U.K. Officials say they are aware of more than 200 people who have gone to Syria from the U.K., and that the number could be much higher.

Denmark At least 45 people linked to Denmark have traveled to Syria since the summer of 2012, Danish authorities say.

France About 110 people from France are believed to be currently fighting in Syria, says the French Interior Ministry. Dozens have returned from combat to France.

Germany At least 210 suspected Islamic extremists, some as young as 16 years old, carrying German passports have left the country to fight in Syria. At least 50 of them have returned to Germany.

Netherlands Up to 100 people have joined the war, according to Dutch officials.

Sweden About 30 or more Swedish nationals have joined al Qaeda-inspired groups in Syria, the Swedish Security Service says.


Write to Siobhan Gorman at siobhan.gorman@wsj.com, Cassell Bryan-Low at cassell.bryan-low@wsj.com and Maria Abi-Habib at maria.habib@wsj.com