Story highlights An ISIS defector says the atrocities by the group were too brutal for him

ISIS militant: "These strikes cannot stop us, our support or our fighters"

He says ISIS has been bracing for airstrikes and had backup locations

The ISIS defector says some Western members were "even more extreme than we are"

The United States may be touting its strikes on ISIS targets in Syria, but one of the terror group's fighters says the hits are trivial at best.

In an exclusive interview with CNN, a Syrian ISIS fighter using the pseudonym Abu Talha said the militant group has been preparing for such attacks.

"We've been ready for this for some time," Abu Talha said. "We know that our bases are known because they're tracking us with radars and satellites, so we had backup locations."

He taunted the U.S.-led coalition that has been pummeling ISIS targets in Syria over the past week, including attacks on mobile oil refineries and vehicles.

"We have revenues other than oil. We have other avenues, and our finances are not going to stop just because of oil losses," the 28-year-old militant said.

"They thought they knew everything. But thank God, they don't know anything. And God willing, we will defeat the infidels."

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Abu Talha said he was among the ISIS fighters who took over Mosul, Iraq's second-largest city, in June.

He said even if coalition attacks impede ISIS, they're not enough to stop ISIS' mission of fortifying an Islamic state across Sunni parts of Syria and Iraq.

"They hit us in some areas, and we advance in others," Abu Talha said. "If we are pushed back in Iraq, we advance in northern Syria. These strikes cannot stop us, our support or our fighters."

U.S. President Barack Obama admitted the quick rise of ISIS in Syria had been underrestimated at first.

In an interview that aired Sunday on CBS' "60 Minutes," Obama said the U.S. government "underestimated what had been taking place in Syria" during its civil war -- allowing the country to become "ground zero for jihadists around the world."

ISIS defector speaks out

One man who recently left the ranks of ISIS also said the militants took precautions before coalition strikes.

"They almost entirely emptied out the headquarters," the defector, using the name Abu Omar, told CNN's Arwa Damon in Turkey. "Some equipment they hid in civilian neighborhoods. Some they hid underground."

Abu Omar also said ISIS relies heavily on foreign members -- including Westerners -- to carry out its mission.

"The French, they have so much control -- they're even more extreme than we are," the defector said. "They come from France, but it's as if they have been part of the 'Islamic State' for years."

The 29-year-old said he believed in ISIS' goal of creating an Islamic state and left his wife and 3-year-old son behind to go fight.

Abu Omar initially tried to defend the group's atrocities -- including crucifixions, beheadings and indiscriminate killings. But eventually, the brutally became too much for him.

"I used to hope that they would fix their mistakes," he said. "If they had, it wouldn't be like this."

When the United States and Arab partners started pounding the ISIS stronghold of Raqqa last week, many militants scattered. And Abu Omar found an opportunity to flee.

But he still trembles when he talks, fearful he has a target on his back.

Abu Omar said while he still dreams of a caliphate in the region, he is haunted by what he witnessed.

"I saw a 70-year-old sheikh killed in front of me," he said. "The 'Islamic State' can't continue like this. ... There are a lot of youth who are joining -- 14, 15 years old. Maybe my voice can make them think again."