WASHINGTON - US Defense Secretary Ash Carter said Wednesday that Europe must accelerate efforts against the Islamic State (IS) extremist group in the wake of terror attacks in Brussels.

"The thing that I think the Brussels event is going to further signify to Europeans is that as we (U.S. military) have been accelerating our campaign to defeat ISIL (another acronym for IS) in Syria, in Iraq and elsewhere, they need to accelerate their efforts," said Carter in an interview with CNN.

Meanwhile, Carter said that the influence of IS has already spilled out of its strongholds in Iraq and Syria.

"It is not enough that we defeat them in Iraq and Syria. What Brussels tell us is that they have sympathizers, people who are Belgians or French, who live there already," said Carter. "Therefore, an important part of the fight is also going to be a homeland security, intelligence and law enforcement fight."

At least 34 people were killed and more than 180 others injured in explosions on Tuesday at the Brussels airport and on a city subway train close to the European Union institutions, according to the latest figures from the Belgian federal prosecutor.

European countries have stepped up security measures and are seeking solidarity in confronting terrorism.