A former top CIA official has warned that the Islamic State (ISIS) is growing stronger and now poses a "significant strategic and lethal threat" to the United States as well as Europe.

Testifying at the House Armed Services Committee on Tuesday, Michael Morell, President Obama's former deputy and acting CIA director, said the number of militant groups now swearing allegiance to ISIS has grown to cover nearly 20 countries, from practically "nothing" a year ago, Fox News reported.

"ISIS has gained affiliates faster than Al Qaeda ever did," he told lawmakers.

Morell made the warning following another ISIS-claimed terrorist attack in Istanbul, Turkey, which killed at least 10 people and wounded 15 others, Turkish officials said.

The former CIA official said the jihadist group has shown time and again its ability to attack targets in Europe. ISIS is "almost certainly" working to do the same in the United States, he said.

Unless they are stopped, he warned, "they will succeed."

A U.S. intelligence official told Fox News the ISIS is continuing to attract fighters from around the world and joining its main force in Syria.

The official estimates that more than 36,000 foreign fighters have travelled to Syria from at least 120 countries in 2015. The figure was nearly double the estimate of more than 20,000 foreign fighters in the previous year, the intelligence source said.

He said this only shows that despite suffering casualties from ground combat and the bombing campaigns led by the U.S. and other countries, the ISIS remains a potent military force with more and more recruits joining the group.

In another demonstration of its global reach, the ISIS said it was behind an attack by suicide bombers and gunmen in the heart of Jakarta on Thursday, the first time the jihadist group targeted Indonesia, the world's most populous Muslim nation, Reuters reported.

"A group of soldiers of the caliphate in Indonesia targeted a gathering from the crusader alliance that fights the Islamic State in Jakarta," the group said in a statement. It did not elaborate.

Only seven people, including an Indonesian and a Canadian, were killed despite multiple blasts and a gunfight. Five of the fatalities were the terrorists themselves. Twenty people, including an Algerian, Austrian, German and Dutchman, were wounded.

"ISIS is behind this attack definitely," Jakarta's police chief told reporters. He named an Indonesian militant called Bahrun Naim as the man responsible for plotting it. Police believe Naim is in the ISIS de facto capital of Raqqa in Syria.