U.S. Secretary of State John Kerry said Wednesday a coalition of air strikes against Islamic State targets in Iraq and Syria are having a "significant impact."

Speaking at a meeting attended by representatives from about 60 nations, Kerry said the U.S.-led coalition has carried out some 1,000 air strikes since September to stop the advancing ISIS militants from taking over huge swaths of territory, Reuters reported.

"It's much harder now than when we started for Daesh to assemble forces in strength, to travel in convoys and to launch concerted attacks," Kerry said, using the Arabic acronym for ISIS, which stands for the Islamic State in Iraq and Syria.

"No large Daesh unit can move forward aggressively without worrying what will come down on it from the skies," the secretary of state said.

"Our impact will be measured most likely in years but our efforts are already having a significant impact."

Kerry's statements, made in Brussels, came as officials in Washington announced Iran recently carried out its own air strikes against eastern Iraq, the BBC reported. However Iran, not a part of the coalition and also Iraq's ally, did not confirm those reports.

Massoud Jazayeri, deputy chief of staff of Iran's Armed Forces, told an Iranian news agency the allegations were "totally untrue" before blaming the U.S. for Iraq's "unrest and problems," the BBC reported.

Back in Brussels, Kerry did not comment on the reports from Washington, but did say that the relationship between the U.S. and Iran has not changed, according to Reuters.

"Nothing has changed in our fundamental policy of not coordinating our military activity, or any other activity, at this moment with Iranians. We're not doing that," Kerry said.

As officials decide on the next course of action against the jihadists, who continue to terrorize Iraqis and publish videos of gruesome beheadings, both Iraq's and Syria's leaders stressed the need for stronger armed forces and ground troops.

"You can't end terrorism with aerial strikes," Syrian President Bashar al-Assad told a French magazine.