PARIS (AFP) -- The Brussels bombings exposed the vulnerability of publically accessible zones in crowded airports, railway stations and subway systems to terror attacks, and it is a risk that can never be completely eliminated, security analysts say.

Islamic State-linked jihadists targeted that weak spot when they detonated bombs that tore through the main hall of Brussels airport and blew up a city-centre metro train, killing about 30 people and wounding more than 200.

The bombers carried out their attacks in high profile transport hubs in the Belgian capital, home of NATO headquarters and the European Union, during a period when the country was under a high security alert just four days after the arrest in Brussels of Salah Abdeslam -- the prime suspect in the Paris attacks four months earlier that killed 130 people.

"While aviation and public transport in general remain an attractive target for terrorist groups, the risk can never be 100 percent eliminated as the threat remains," said Ben Vogel, editor of IHS Jane's Airport Review.

The challenge of stopping an attack is greater when the perpetrators target unsecured areas such as the check-in area of the departure hall in Brussels' Zaventem airport, prior to the passport and baggage controls, he said.

"This causes a headache for security agencies that have focused primarily on the passenger screening checkpoint, with some success as terrorist organisations seem to have altered their modus operandi," Vogel said.

In Europe, most airport arrival and departure halls are freely accessible outside of the secured zone that lies beyond baggage, passport and customs controls.

'Targets for terrorists' 

Security at airport arrival, departure and check-in areas can be improved, experts said, but it is difficult to restrict access without paralysing ever larger airport terminals.

"You have to be rational and seek maximum efficiency," said French Interior Minister Bernard Cazeneuve.

"If you set up checks at the entrance to airports you going to clog up the airports, you are going to make it impossible for the economy to work," Cazeneuve said.

"If you create queues in front of airports, you also create targets for terrorists," he added.

Extremists will always find a weak spot, said David Bentley, British aeronautics expert, in a written reply to questions from AFP.

"They will attack people arriving in vehicles, on public transport etc.," Bentley said.

"The logistics of locking down an airport like that is contrary to the free-flow of traffic by which it sustains itself. Imagine if cars were stopped kilometres from the airport to be searched. Where, exactly, without causing huge traffic jams?" Bentley added.

"Realistically, only better intelligence will prevail."

French lawmaker, Gilles Savary, proponent of a new transport security law passed by parliament this month, said the aim was to "tighten the net as far as possible, knowing that someone can always get through".

"If Zaventem was an inaccessible bunker they would have hit a market in Brussels. So we have a choice between 'bunkerization' and daily life, and between the two there is a risk," he told AFP.

Europe is the "big missing piece" in transport security, Savary said, urging the bloc to enact ground transport security legislation such as that implemented in the air industry after the September 11, 2001 attacks on New York and Washington.

Trains are the big challenge

A foiled jihadist attack in August last year on a high-speed Amsterdam-Paris train operated by Thalys, followed by the November 13 Paris attacks, prompted Thalys to install passenger metal detectors and baggage X-ray scanners for access to its platforms in December.

France's national railway operator SNCF said this week that it spends more than 400 million euros ($450 million) a year on security.

The SNCF is also developing new systems to detect explosives and weapons, as well as behaviour analysis software, in coordination with the French regulator of personal data protection, management said in an email to AFP.

Extending the high security perimeter right up to the entry and exit points of airport terminals would require a complete reconfiguring of terminals at a high financial cost and would mean even longer waits for passengers, said Vogel of IHS Janes.

"The security challenge for land transport, particularly rail modes, is even harder," Vogel added.

"The cost of securing their rail networks is so daunting that the 28 member states have blocked the Commission for years from making any proposals at the EU level regarding rail security.  Whether that blockage can hold against public pressure for action is the test for the coming months."