US-led airstrikes against Islamic State in Iraq have killed three of the militant group's top leaders, the head of the US Joint Chiefs of Staff told the Wall Street Journal on Thursday.

In an interview with the newspaper, US General Martin Dempsey said the senior leaders were killed in recent weeks as part of expanding effort with partner nations to combat the militants.

US-led air strikes in Syria focused on the contested city of Kobane near the Turkish border, with six air strikes that destroyed seven Islamic State fighting positions, a building and a tactical unit, the US military said on Thursday.

US and partner nations also conducted five air strikes in Iraq on Thursday, targeting Islamic State militants in Tal Afar, Mosul and Ramadi, the US Central Command announced in a statement.

*This story was edited by Ahram Online.