PARIS • The Islamic State in Iraq and Syria terror group previously cited traditional Christmas markets as viable targets in its wave of terrorism in Europe, and the Berlin assault was reminiscent of July's truck-on- sidewalk attack in Nice.

The United States State Department in fact issued a specific travel warning to Americans last month, citing "credible information" that Islamists were targeting holiday season events.

Yet, the Christmas market attacked on Monday appeared to lack basic protections, such as concrete barriers, to ward off a Nice- like attack.

Countering the criticism, Berlin police chief Klaus Kandt said: "We cannot turn Christmas markets into fortresses."

Indeed, even the best-laid security plans cannot protect an open-air event against the lethal potency of a truck attack, experts say.

The sheer scope of places, demonstrations and public gatherings needing protection is such that the risk of someone using a moving vehicle as a weapon of terror will remain high for years.

On Monday night, a lorry ploughed into a busy Berlin Christmas market, killing 12 people in what German Chancellor Angela Merkel called a terrorist attack.

On July 14 in Nice, France, a militant drove a 19-tonne truck onto the pavement, careening through police barriers and into crowds leaving a fireworks display, killing 86.

"The only events that can be protected are those that are planned in advance, where you can take measures like closing off streets or putting down concrete blocks," a senior anti-terrorism official in France, who asked to remain anonymous, told Agence France-Presse.

And even then, security measures that include stationary protective devices such as concrete barriers can be circumvented.

In the wake of the Berlin carnage, security has been ratcheted up across Europe, especially at holiday Christmas markets from Austria to Britain. But it is not just public festivities that are at risk, according to the anti-terrorism official, noting that a rampaging vehicle could cause mayhem on a city street any day of the week.

"There is nothing you can do... That is the whole problem with these attacks which take the form of sudden strikes on soft targets that are easy to carry out," he said.

Mr Ludovic Guerineau, former head of the French external intelligence service DGSE and who now heads operations at private security firm Anticip, said: "We need a serious risk assessment to start with, and that costs money."

Noting that easily accessible areas and natural environments are especially vulnerable, he said: "They require structural modifications."

In 2010, terror group Al-Qaeda's online propaganda magazine Inspire carried step-by-step instructions on how to use pickups in ramming attacks.

In an article, titled "The Ultimate Mowing Machine", it advised attackers to use a big vehicle - "the stronger the better".

"In future, when organising an event that draws a crowd, you will have to consider this threat," the French former spy chief said.

"But when it is not planned, there is nothing to be done. We cannot ban all the trucks from our cities."

AGENCE FRANCE-PRESSE,

WASHINGTON POST