BRUSSELS — A top European security official warned Thursday that the threat of Islamic State attacks is greater than previous assessments, underscoring calls for tighter security, as police widened the hunt for accomplices in the Brussels blasts.

Rob Wainwright, chief of Europol, said the terrorist group, which is facing battlefield setbacks in Iraq and Syria, has adopted a ‘‘more aggressive’’ posture toward Europe.

Wainwright said security authorities are focused on about 5,000 suspects who had become radicalized in Europe and traveled to Syria to fight. Many have now returned.

‘‘We are faced by a more dangerous, a more urgent security threat from so-called Islamic State,’’ Wainwright told the BBC on Thursday. ‘‘It threatens not just France and Belgium but a number of European countries at the same time. . . . It is certainly the most serious threat we have faced in at least a decade.’’

Wainwright spoke before an emergency session of European security ministers in Brussels. Europe’s leaders have been criticized for not acting more quickly to integrate security strategies, and are under pressure now to produce results.

In particular, Belgium has come up short in its efforts to prevent extremist attacks time and again, security analysts say — failing to coordinate intelligence, investigate suspects, and control its borders.

Historically, the country has often been found wanting when it comes to sharing intelligence among agencies, applying what’s learned to police work, and controlling its external borders, Michael O’Hanlon, a senior fellow at the Washington-based Brookings Institution, told the Associated Press.

In addition, he said, Brussels often fails to strike the balance that other countries achieve in weighing the desire to investigate suspected criminal activity and the need to act quickly when an immediate threat is identified.

“I suspect they will rethink things from first principles now,’’ O’Hanlon said.

In a shocking departure from the habitually polite speech of European relations, French Finance Minister Michel Sapin accused Belgian officials of a ‘‘lack of will . . . maybe also a kind of naivete’’ in ignoring the spread of radical Islam among the country’s 650,000-member Muslim population.

Alain Marsaud, a conservative member of France’s Parliament, said in a newspaper interview he was ‘‘disgusted by the inability of the Belgians over these recent months and years to deal with this problem.’’

He expressed particular astonishment that it took the Belgians more than four months to capture Paris attacks suspect Salah Abdeslam, who returned to Brussels the day after the bloodbath in the French capital but eluded Belgian authorities multiple times and was run to ground only Friday.

At their emergency meeting in Brussels Thursday, European Union justice and interior ministers discussed the Brussels attacks that killed 31 people and injured about 270 Tuesday.

The ministers pledged to cooperate more closely on intelligence sharing. They also appealed ‘‘as a matter of urgency’’ for the European Parliament to adopt an agreement that would allow authorities to exchange airport passenger data.

The passenger data issue has long disturbed privacy campaigners, and figures large in the debate over security versus liberties. The United States has long pushed for better data sharing, but Europeans have balked over privacy issues.

Raffaello Pantucci, director of international security studies at the Royal United Services Institute in London, called it ‘‘depressingly predictable’’ that a major attack would occur in Brussels.

‘‘There is sort of a perfect union,’’ he said — a combination of homegrown, hardened Muslim radicals willing to act and possessing the tools and opportunity, as well as a government and law enforcement structure that simply isn’t up to the task.

Among the Belgian vulnerabilities: Guns, including illegal battlefield-grade weapons from the former Yugoslavia, are readily available.

In addition, a complicated, disjointed governmental structure has hindered the forging of a unified front against extremism. Mayors in the greater Brussels area complained last year that even when officially alerted to the presence of suspected radicals in their municipalities, they lacked the power to do anything about it.

The Brussels area, a mosaic of 19 municipalities where 1 million people live, has six police zones, compared with a single law enforcement agency for all of New York City and its population of 8.4 million.

And around the country, different forces operate in French, Dutch. or German, complicating communication.

All that has allowed radical groups to operate with less fear of detection than they face elsewhere. Per capita, Belgium is the Islamic State’s most fertile recruiting ground in Europe as it seeks to find Westerners to fight for its self-styled caliphate in Syria and Iraq.

Many of the suicide bombers and gunmen in last year’s Paris attacks lived in Brussels. And French authorities say Belgian involvement is suspected in the abortive attack on an international express train and a failed plot to attack a Paris-area church.

Last month, the Belgian government announced a $450 million program to combat ‘‘terrorism and radicalization,’’ including the hiring of 1,000 additional police, prosecutors, state security agents and other personnel.

Material from the Associated Press was included in this report.