The Islamic State’s web of affiliates is growing faster than Al Qaeda’s ever did, a former top CIA official told Congress on Tuesday – on the heels of yet another attack blamed on ISIS.

Michael Morell, President Obama’s former deputy and acting CIA director, issued the warning in testimony to the House Armed Services Committee.

He said the number of militant groups now swearing allegiance to ISIS has grown to cover nearly 20 countries, from practically “nothing” a year ago.

“ISIS has gained affiliates faster than Al Qaeda ever did,” Morell said.

He testified after a suicide bomber, whom Turkish officials say was affiliated with ISIS, detonated a bomb in the tourist heart of historic Istanbul. The attack killed at least 10 people -- nine of them German tourists -- and wounded 15 others, Turkish officials said.

German Chancellor Angela Merkel vowed to continue the fight against terror after the attack. "Today Istanbul was the target, before Paris, Copenhagen, Tunis, and so many other areas," she told reporters in Berlin, listing the network’s recent roster of terror targets.

In Washington, Morell said the group now poses a “significant strategic and lethal threat” to the United States, as well as Europe. Noting that the group has demonstrated the ability to attack targets in Europe, Morell said ISIS is “almost certainly” working to do the same in the United States.

Unless they are degraded, he said, “They will succeed.”

The group’s affiliate network is growing as it continues to attract fighters from around the world to the belly of the beast – in Syria.

A U.S. intelligence official told Fox News that more than 36,000 foreign fighters – at least 6,600 with Western passports – have traveled to Syria from at least 120 countries. That’s up from an estimate of more than 20,000 foreign fighters last year.

The numbers reflect that the group, despite suffering casualties, continues to weather the bombing campaign led by the U.S. and other countries – and attract more recruits.

President Obama, in his final State of the Union address Tuesday night, is expected to discuss his plans and strategy for confronting ISIS.

Fox News’ Lucas Tomlinson and Catherine Herridge and The Associated Press contributed to this report.