Airstrikes south the Iraqi city of Fallujah on Wednesday may have killed up to hundreds of "Islamic State" (IS) fighters. US officials estimate that at least 250 IS fighters were killed and at least 40 vehicles destroyed.

The estimate is preliminary, but if confirmed it would amount to one of the deadliest attacks against the jihadi group. US media reported that American airstrikes were responsible for the attack, while the BBC said the Iraqi air force bombed the IS fighters.

The attack is the latest military set back for IS inside their self-proclaimed "caliphate," which spans parts of Iraq and Syria.

But territorial loses haven't diminished the group's ability to strike abroad.

Battle against IS will be long

Wednesday night's terror attack in Istanbul, which killed more than 40 people and injured nearly 250 more, is being blamed on IS. Although the militant group has not claimed responsibility for the attack, both Ankara and Washington have said the attacks bears the hallmarks of an IS operation.

In Washington, CIA chief John Brennan said the battle to defeat IS remains far from over, despite recent gains in the battlefield. That includes US-backed Iraqi forces liberating the strategic city of Fallujah last week.

"We've made, I think, some significant progress, along with our coalition partners, in Syria and Iraq, where most of the ISIS members are resident right now," Brennan said, referring to the militant group by an alternate acronym.

"But ISIS' ability to continue to propagate its narrative, as well as to incite and carry out these attacks - I think we still have a ways to go before we're able to say that we have made some significant progress against them."

Another US-backed alliance of militias in Syria have launched a major offensive against IS in a bid to take back the city of Manbij in northern Syria.

But overall it remains a back-and-forth conflict. US-backed Syrian rebels were reportedly pushed back from the outskirts of an IS-held town on the border with Iraq on Wednesday when jihadists mounted a counter-attack.

bik/sms (Reuters, dpa)