Washington has been outraged by the dozens of rocket attacks and roadside bombs targeting its mission and other US military installations across Iraq over the last year.

The partial withdrawal appears to be the result of fresh security concerns, one Iraqi official told AFP.

"It's a minor drawdown based on security reservations from the US side. They could come back -- it's just a security blip," the senior source said, speaking on condition of anonymity in order to talk to the press.

"We knew ahead of time and top diplomatic staff including the ambassador are staying, so this is not a rupture of diplomatic ties."

A second top official confirmed it was an effort to "minimise risk."

Neither could say how many of the several hundred diplomats based at the embassy had been pulled out.

A US State Department spokesperson declined to comment on the withdrawal but said the safety of US officials, citizens and facilities in Iraq "remains our highest priority".

The spokesperson said US ambassador Matthew Tueller was still in Iraq and that the embassy "continues to operate".

Washington has blamed the rocket and roadside bomb attacks on pro-Iran hardline groups in Iraq and has retaliated twice by bombing one of those factions, Kataeb Hezbollah.

When the attacks continued, the US issued an ultimatum to Iraq, threatening to fully close down its embassy.

That prompted Iran-aligned factions to agree to a "truce" in mid-October, and the attacks swiftly stopped.

On November 17, a volley of rockets hit several Baghdad neighbourhoods, killing one girl.

Top Iraqi and Western officials said at the time that they expected the truce to hold, but said Washington was still drawing up withdrawal plans.

One Western official told AFP in late November that the US was studying three options, including a partial withdrawal.

"They're exploring drawing down the embassy to just the ambassador and key diplomatic staff," the official said.

Iraqi and Western officials see a turbulent few weeks ahead of the White House handover from President Donald Trump, who has pursued a "maximum pressure" policy against Iran that has also squeezed its allies next door in Iraq.

They did not rule out last-minute military action by the Trump administration on Iranian interests in Iraq.

"There's a feeling that there are a few weeks left to go (before Trump leaves office) and who knows what could happen," the Western official said.