The threat of terrorists attacks in the EU remains the top national security concern among member states, with some 211 foiled plots last year alone, according to a top EU security official.

The threat of another attack similar in scale to the Paris attack in November last year or in Brussels this past March has national authorities on edge.

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Manuel Navarrete Paniagua, the head of the European Counter Terrorism Centre at the EU police agency Europol, told euro-deputies on Monday (23 May) that terrorist cells in the EU were likely stockpiling explosives for later use.

"We have some information reported by the member states that terrorists groups are trying to establish large clandestine stockpiles of explosives in the European Union to be used eventually in large scale home attacks," he said.

Paniagua was briefing MEPs in the civil liberties committee on the yet-to-be published EU Terrorism Situation & Trend Report (TE-SAT) compiled by Europol and member states.

The report is set for publication in a few weeks' time.

Paniagua cited figures and broader conclusions due in the report, noting that "jihadist terrorism" remains the top threat to security in the European Union.

The latest attacks in the EU suggest much better coordination among the terrorists than previously thought. He said their combined use of explosives and firearms was also a novelty and a threat that is evolving rapidly.

Authorities have yet to nail down any single profile of a home-grown terrorist or foreign fighter.

But he said many have petty criminal backgrounds. He pointed out a significant proportion of foreign fighters have also been diagnosed with mental problems.

More than 4,000 so-called foreign fighters have been identified in the EU and entered into a Europol database.

"Using the terrorist financial tracking programme, we provided last year more than 2,700 leads regarding foreign terrorist fighters to the member states," he said.

Some 1,057 people were also arrested last year on terrorism-related offences, he said.

He said there are also unconfirmed reports that some jihadist militants have set up training camps in the Western Balkans and some EU states.

Terrorists and migration flows

Other concerns related to terrorists exploiting the migration flows from Turkey and elsewhere to enter the European Union.

"We found no evidence of the systematic of using this flow to infiltrate terrorists into the European Union. But they do, they use it, we have some cases, some of the people that perpetrated the Paris attacks were eventually disguised in this immigration flow," he said.

Paniagua said migrants were being victimised twice as result. First they flee terrorists and then again when the same attempt to infiltrate the EU by hiding among the refugees, he said.

"Again, we have no indication that this systematic," he said.

Earlier this month, Europol announced it would deploy some 200 counter-terrorist and other investigators to migrant arrival centres known as hotspots in Italy and Greece.

Of those, Europol said it would dispatch around 50 "at key points on the external border of the EU" to track down and help identify suspected criminals and terrorists.

A European Commission report earlier this month noted that Turkey is suspected to have around 1,300 Turkish citizens fighting alongside Islamic militant groups in Syria and Iraq.

It noted that the plan to lift visa restrictions on Turkish nationals "could potentially have an impact on the terrorist risk in the EU".