The number of people arrested in Europe on suspicion of Daesh terror activities nearly doubled over the last two years, Europe's policing agency said Thursday, adding that actual attacks have decreased.

Some 718 were arrested on offenses relating to Daesh terror last year as opposed to 395 in 2014, while terrorist attacks dropped from 17 in 2014 to 13 last year -- of which six were linked to the Daesh terrorist group, Europol said in a new report.

Europe had experienced an "unprecedented form of terrorist attacks," said Europol director Rob Wainwright.

The biggest threat was said to come from lone attackers returning from countries such as Syria, Europol said.

Groups like Daesh were very successful online and had made "optimal" use of social networks for propaganda and recruiting, the report said.

Some 142 terrorist attacks had been registered in 2016 in Europe, the report said, adding that nationalist and separatist groups had been behind most of the attacks.