Easy to organise, hard to prevent

Militants are increasingly turning to vehicle-ramming attacks, like the one staged near Britain's parliament on Wednesday, because they are cheap, easy to organise and hard to prevent.

Experts say the tactic of mowing people down avoids the need to obtain any explosives or weapons and can be carried out by a "lone-wolf" attacker without using a network of fellow militants - all lessening the risk of alerting security agencies.

"This kind of attack doesn't need special preparation, it is very low-cost, within anybody's reach," said Sebastien Pietrasanta, a French Socialist lawmaker and terrorism expert.

"It is often a case of individual action," he told Reuters. "They can be quite spontaneous."

Four people were killed and at least 20 injured in London after a car ploughed into pedestrians and an attacker stabbed a policeman close to parliament in what police called a "marauding terrorist attack". The attacker was shot dead. Islamic State claimed the attack.

Trucks were used to devastating effect last year against crowds in Berlin and Nice, in contrast to more organised attacks that have already hit Paris and Madrid - as well as London in 2005 - using teams of bombers or gunmen.

Islamic State claimed responsibility for both the Nice attack last July, when a truck killed 86 people celebrating Bastille Day, and for the Berlin attack in December, when a truck smashed through a Christmas market, killing 12 people.

IS encouraged readers of its online magazine Rumiyah in 2016 to use vehicles to kill and injure.

Vehicle attacks are not a new tactic in the Middle East. Palestinians often use car to ram Israeli targets.

Jean-Charles Brisard, president of the Centre for the Analysis of Terrorism, a European thinktank, said Wednesday's attack seemed to be "rudimentary in its conception".

Using a car as a battering-ram was a tactic that was highly rated by militants because it was lethal, he said. "With a vehicle, they cause a lot more deaths than with a knife or a machete."

Tyson Barker, programme director with the Aspen Institute thinktank in Germany, said the London attack underscored the difficulty of protecting "soft" targets, and the trade-offs between security and liberty in open Western societies.

Saudi Arabian Interior Ministry spokesman Mansour al-Turki, part of a delegation in Paris for talks with French officials on counter-terrorism, said the defeat of groups like Islamic State and al-Qaeda could lead to a splintering of the threat, creating new problems for governments.

"When they are defeated in Syria and Iraq we are all going to face difficulties and nobody knows where the ISIS (Islamic State) fighters will go to," he told reporters.

"I think we will be entering the next phase of terrorism which is through social media and lone wolves," he said.