WASHINGTON — U.S.-led military efforts to contain violent Islamic State radicals in the Middle East have not yet succeeded, and the group’s engine of worldwide terrorism “still has a lot of momentum,” CIA Director John Brennan said.

“We cannot rest at all. We have to increase our efforts,” Brennan said.

Brennan said suicide bombings at Istanbul’s international airport a day earlier, which killed 41 people, bore “the hallmarks” of the Islamic State.

He described the group as far larger than al-Qaida ever was and determined to attempt to strike terror inside the United States.

“It would be surprising to me that ISIL is not trying to hit us both in the region as well as in our homeland,” Brennan said in a talk at the Washington office of the Council on Foreign Relations, a research center. He used an alternate abbreviation for the group.

The Islamic State, which also is referred to as Daesh, seeks to inspire homegrown extremists to launch attacks, he said.

While U.S. border defenses were strengthened considerably after the 2001 terror attacks on New York and Washington, Brennan said: “If anybody here thinks that the U.S. homeland is hermetically sealed and that Daesh or ISIL would not consider that, I would guard against that.”

Brennan cautioned against thinking suicide bombings can be routinely thwarted.

First off, he said, suicide bombers “don’t have to worry about an escape route. It makes carrying out that attack so much easier.”

He noted the terrorists use modern encryption methods that facilitate communications, and generally operate with only a handful of individuals.

Turmoil in the Middle East, which has left war-torn states such as Yemen, Syria, Iraq and Libya devastated, has provided fertile ground for the Islamic State’s growth, he said.

“Al-Qaida at its height had maybe 2,000 individuals with maybe a core of several hundred. ISIS has tens of thousands of individuals that are scattered not just in the Middle East but also to West Africa, to Southeast Asia and beyond,” Brennan said.

On broader issues, Brennan said worldwide turmoil was rising.

“Global instability is one of the defining issues of our time, and its implications are hard to overstate,” Brennan said.

On other matters, he said:

• Britain’s pending exit from the 28-nation European Union sends the continent into a period of deep uncertainty, but the tumult “will not adversely affect the intelligence partnership” between Washington and London.

• The human toll from rising instability is vast, including a United Nations estimate last week that conflict has displaced 65 million people from their homelands.

• Attacks on the internet are coming from a broader range of actors, including foreign governments, criminal gangs, extremist groups and activists, shaping a panorama in which attackers “can penetrate a network and withdraw in very short order, plundering systems without anyone knowing they were there until maybe after the damage is done.”