Terror networks such as the Islamic State group are evolving their tactics to attack soft targets in Europe, which could see the use of deadly car bombs, Europol warned.
Jihadist attacks on EU member states have not yet involved "the use of home-made, commercial or military explosives in vehicle-borne improvised explosive devices" as in Syria or Iraq, Europe's police agency said in a report issued in The Hague.
But "given the fact that the (techniques) used in Middle Eastern countries tend to be copied by terrorists operating in Europe... it is conceivable that jihadist groups will use this means at some stage," it said.
The group responsible for attacks in Paris a year ago and in Brussels in March wanted to deploy such devices until police actions forced them to change their plans, the report said.
Jihadists massacred 130 people in November 2015 in attacks on the Bataclan concert hall, a handful of bars and restaurants in eastern Paris and France's national stadium.
In Belgium, suicide bombers struck Brussels airport and a metro station near the European Union headquarters on March 22, killing 32 people.
The 14-page report, an update on methods and tactics used by IS, also said counter-terror experts were concerned that strife-torn Libya could develop into a "second springboard for IS, after Syria, for attacks in the EU and North Africa."
Since the armed revolt five years ago that overthrew dictator Moamer Kadhafi, the North African country has been plagued by violence and political instability.
"Experts expect that IS will start planning and dispatching attacks from Libya if the current phase, in which they are primarily focused on taking territory and dispatching of local enemies, comes to an end."
Europe's security services arrested 667 suspects for suspected jihadist activities in 2015, the report added.
A Belgian mayor said he had been told that his country was under threat again, following the March 22 suicide bombings at Brussels airport and on the metro.
"It's new for Belgium, it's new for Europe... that there is a call from the highest level of IS on using car bombs in Europe," Vilvoorde Mayor Hans Bonte, who has contacts with Belgian intelligence and police, told AFP.
"Belgium is clearly mentioned as one of the target countries," added Bonte, whose town just north of Brussels has seen a number of young people leave to fight jihad in Syria.
A Belgian official told AFP on condition of anonymity that it is likely that the Belgian government will issue new detailed instructions over the next few days to police agencies about how to deal with the risk of car bombs.
The official also said the alert level will stay at three, the second-highest level in Belgium, not because of the new car bomb threat but because there are several large upcoming public gatherings for Christmas and other seasonal events.