Such documents represent an additional security risk for European states grappling with a large influx of refugees from countries including Syria and Iraq because they 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 ISIL.
The diplomat said there were around 5,000 missing Syrian passports from Raqqa and Deir Ezzour provinces on the list and 10,000 Iraqi ones from Anbar, Nineveh and Tikrit. It was not immediately clear which authorities had originally provided the serial numbers.
Meanwhile, the US government has warned that ISIL has the ability to create fake Syrian passports, in a report that appears to draw on the same information described by the diplomats.
The extremist group has access to Syrian government passport printing machines and blank passports, the report said, adding that it was possible individuals from Syria may have travelled to the United States on passports “issued” by ISIL.
The passport watch list was first shared between several European Union states in the summer, when tens of thousands of refugees started arriving in Europe, and has been updated several times since, diplomats said.
The debate on how to deal with the influx of refugees to Europe has become more politically charged after deadly attacks in Paris that stoked fears ISIL militants could exploit the migrant crisis to send extremists to Europe.
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 Greece in October, although it has not been confirmed that he was the man named in the passport.
In Switzerland, authorities said on Saturday that two people of Syrian origin had been arrested as part of an investigation into a terror threat.
The two suspects were arrested on Friday on suspicion of “manufacture, concealment and transport of explosives and toxic gases” and violating Swiss law prohibiting “groups like Al Qaeda, Islamic State and similar organisations”, said the federal prosecutor’s office.
Also on Saturday, Russia slammed as unrepresentative the talks held this week in Saudi Arabia between Syrian political and armed opposition groups, which culminated with a call for president Bashar Al Assad to step down.
“We cannot agree with an attempt made by the group that gathered in Riyadh to monopolise the right to speak on behalf of the entire Syrian opposition,” said the Russian foreign ministry, which supports Mr Al Assad.
Moscow said the talks did not include members of the so-called “patriotic Syrian opposition” seen as friendly towards Mr Al Assad, and also criticised the opposition groups for continuing to insist on a “number of preconditions”.
On the ground in Syria, 16 people were killed and dozens wounded when a large car bomb exploded near a hospital in the central city of Homs on Saturday. The blast hit the neighbourhood of Al Zahraa, where most residents belong to the same Alawite sect as Mr Al Assad.
Reuters, Agence France-Presse