The European Commission is scrambling experts from member states to meet in Brussels following traffic chaos and huge traveller delays over new security rules at the external borders.

"When it comes to lengthy queues we are in contact with several member states and we are going to have an expert meeting this week to address these issues," an EU commission spokesperson told reporters on Monday (10 April).

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The rules are part of a larger anti-terror effort after EU nationals that had fought alongside militant extremists in Syria then returned to launch attacks in Paris.

National authorities are required as of last week to ID check, using security databases, every EU citizen that leaves or enters the outer fringes of the Schengen area of 26 EU states.

The task is large. In 2015, more than 200 million border crossings were registered at the external borders of the Schengen area.

Within hours of its launch, Slovenia suspended the checks, due to the massive traffic jams at its border with non-Schengen EU member state Croatia.

Huge delays were also reported at airports in Greece and Spain.

The Greek police announced they would suspend the systematic ID checks of EU citizens for six months.

Outdated ID cards of Greek nationals meant border authorities had to manually enter data, according to Greek media news outlet Kathimerini.

The rules allow for a six-month derogation at airports but not at land and sea borders, posing questions on why people had to endure long waits at the border between Croatia and Slovenia.

The EU commission on Monday said member states "did indeed have time to prepare for the entry into force of these new rules".

The proposal for systematic checks was first announced in December 2015 and then adopted by EU countries early last month.

The commission says authorities are allowed to derogate from the checks at land and sea borders, but must first provide "a thorough risk assessment" to make sure the overall level of security is not weakened.

EU home affairs commissioner Dimitris Avramopoulos is in talks with the Croatian authorities over the affair.

Other EU commission officials are also talking to the Slovenians.

Schengen information System

Broader concerns remain over some of the security databases used during the latest checks.

Among those is the EU-wide Schengen Information System (SIS) that issues alerts to border authorities on everything from missing persons to wanted criminals.

Authorities can also issue alerts on foreign fighters, although that data has not always been entered into the system.

Proposals, which are yet to be adopted, have since been floated to make it mandatory.

Gilles de Kerchove, the EU's anti-terror coordinator, last year said only five EU states entered information on foreign fighters in Europol's database, while most others enter alerts in SIS.

He noted that the information on the homegrown returning terrorists is also often incomplete.

The EU commission had also downplayed the threat of returning fighters.

EU commissioner for security Julian King last November described it as a speculation that "is sometimes exaggerated."

This article was corrected on Wednesday (12 April) at 13.54 to state that 5 EU states enter information into a Europol database, not SIS.