Iraq's security situation is likely to see "turbulence" as American forces depart and groups including Al-Qaeda seek to take advantage of this, the top US general in the country said on Monday.

General Lloyd Austin, the commander of United States Forces - Iraq, also said that while Iraqi security forces have generally proven competent in internal security, they still have a long way to go on external defence.

"I think as we leave, you can expect to see some turbulence in security initially, and that's because you'll see various elements try to increase their freedom of movement and freedom of action," Austin told reporters in Baghdad.

"Al-Qaeda will be one of those elements," he said.

"We expect that Al-Qaeda will continue to do what it has done in the past," he said, referring specifically to the situation in northern Iraq.

"We expect that it's possible that they could even increase in their capability.

"Of course, that will depend on how effectively the Iraqi security forces and the government of Iraq are able to focus on that network."

Austin also pointed to Iranian-backed militias as a threat to stability.

"When you look at the environment in the south, we've seen activity over the last several months that are from the Iranian-backed militants," he said. "We expect that that type of activity could possibly continue into the future.

"The Iraqi government has to treat them (Shiite militias) based upon what they really are, and again, these are elements that are really focused on creating a Lebanese Hezbollah kind of organisation in this country," he said.

That means "a government within a government, and those elements would have their own militia, that sort of thing. I think as we leave, if these elements are left unchecked, they will then eventually turn on the government."

Austin also said that significant work remains for Iraq's security forces when it comes to external defence.

"They're approaching having the ability to control the internal security environment," Austin said. But "I don't think they have very much of a capability at all to address an external threat.

"They've been focused on internal defence throughout, and quite frankly, to be fair, they've been fighting ever since we stood up the first platoon of troops here, and we've grown significant capability over the last eight and a half years," he said.

"But because of the fact that we've been focused on internal security, we really haven't had the ability to focus in earnest on developing a foundational capability to defend against an external threat, and that's really what they need to begin to focus on in the future.

"We really intend to remain engaged with Iraq," Austin said. "There's likely to be some setbacks, some tough days ahead, but I am very hopeful that we'll stay on course."

US President Barack Obama announced on October 21 that US troops would depart Iraq by year's end.

Austin said on Monday that there are fewer than 20,000 US soldiers left in Iraq, while eight military bases remain to be handed over.