Iraq's defence chief has said his military will not be fully ready to defend Iraq from external threats until 2020 to 2024, according to a US inspector's report released on Sunday.

Lieutenant General Babakir Zebari has repeatedly warned that Iraq's security forces, rebuilt after the 2003 invasion that ousted strongman Saddam Hussein, would not be ready for years.

US President Barack Obama announced on Oct. 21 that American troops would fully withdraw from Iraq by year-end, as scheduled under a 2008 security pact between the two countries.

Both Iraqi and US military leaders have said the army and police are capable of containing internal threats from Sunni insurgents and Shi'ite militias that launch scores of attacks monthly, but that they lag in external defence.

"General Zebari suggested that the MOD (Ministry of Defence) will be unable to execute the full spectrum of external-defence missions until sometime between 2020 and 2024, citing ... funding shortfalls as the main reason for the delay," said the report from the US Special Inspector General for Iraq Reconstruction (SIGIR).

Zebari said the air force would not be able to defend Iraqi airspace until 2020 and is not capable of supporting ground combat operations, citing a long-delayed deal to buy F-16 warplanes from the United States, the SIGIR report said.

"An army without an air force is exposed," the report quoted Zebari as saying.

Iraq delayed its purchase of F-16s earlier this year to divert money to social programmes.

Officials said in late September that Iraq had signed a deal to buy 18 of the combat jets. The first delivery is not expected for several years.

Washington has around 39,000 troops still in Iraq, down from a peak of about 170,000 during the war. Violence has dropped sharply from the sectarian bloodbath of 2006-07 when tens of thousands died.

As it tries to reintegrate itself into the region after years as a pariah, Iraq is warily eying neighbours such as Iran, Saudi Arabia, Turkey and Syria.

Iraqi leaders have accused neighbours of meddling, and US military officials say Iran arms Shi'ite militias in Iraq.

"While we have no enemies, we also have no real friends," the SIGIR report quoted Zebari as saying of the Iraqi government's relations with its neighbours.