US-led air strikes against ISIL in Iraq have killed several of the group's senior leaders, but not the group's top commander, Abu Bakr al-Baghdadi, US officials have said.

Since mid-November, coalition air strikes killed "multiple senior and mid-level leaders" within the Islamic State of Iraq and the Levant (ISIL) group, Pentagon Press Secretary Rear Admiral John Kirby said in a statement on Thursday.

"We believe that the loss of these key leaders degrades ISIL's ability to command and control current operations against Iraqi Security Forces, including Kurdish and other local forces in Iraq," the statement added.

A Defence Department official speaking to Al Jazeera said those killed included Haji Mutazz and Abd al-Basit, who were described as high-level leaders within ISIL.

The official said a mid-level commander, Radwan Taleb al-Hamdouni - believed to have been instrumental in ISIL's takeover of Mosul - had also been killed.

The announcement of the killings came as the head of the coalition campaign against ISIL, US Lieutenant General James Terry, hailed the impact of four months of air strikes in Iraq.

"We've made significant progress in halting that [ISIL] offensive," Terry told reporters.

The US and allied aircraft have carried out 1,361 raids against ISIL since bombing began on August 8, Terry said.

He said air strikes this week around the Sinjar Mountains helped Kurdish Peshmerga forces recapture a large area near the Syrian border.

Masrur Barzani, chancellor of Kurdistan Region Security Council, said the Peshmerga had managed to establish a passageway to the Sinjar Mountains so that thousands of people from the country's Yazidi minority who have been trapped there for months can flee.

The capture of Sinjar by ISIL fighters in early August and the plight of the mostly Yazidi population there was cited by President Barack Obama as one of the reasons for the US military intervention.