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."
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."