The head of the European Union's border agency has said the large number of refugees entering Europe poses a security risk, with civil war making it harder to check the authenticity of Syrian passports.
Hundreds of thousands of people fleeing war and poverty in countries such as Syria and Iraq have arrived in Europe this year. Since last month's attacks in Paris, concern has grown that Islamist militants could enter undetected among the influx.
"The big inflows of people who are currently entering Europe unchecked are of course a security risk," Frontex head Fabrice Leggeri told German newspaper Welt am Sonntag, though he also said it would be wrong to regard all refugees as "potential terrorists."
In Paris, a Syrian passport was found near the dead body of one of the attackers and his fingerprints matched those of a person registered as arriving in October in Greece, the main entry point for refugees and migrants.
Leggeri said that in a country going through civil war like Syria no one could guarantee that "the documents that look real are really issued by an official authority or are really being carried by their rightful owner."
Diplomats said earlier this month European countries have circulated a watch list of missing Syrian and Iraqi passports they fear could be filled with false data and used by people to travel to Europe and beyond. Such documents are harder to identify than outright fakes.
One diplomat said the list contained serial numbers of thousands of genuine blank passports that were held in government offices in parts of Syria and Iraq that have since been captured by armed groups including Islamic State.
Welt am Sonntag said that according to Western intelligence services, Islamic State militants had probably procured tens of thousands of real passports after taking control of local authorities in parts of Syria, Iraq and Libya. They may also have seized machines used to produce identity documents.
The newspaper said, without specifying its sources, that Islamic State was doing a brisk trade in the passports, which sell for between 1,000 and 1,500 dollars on the black market.