The US State Department ordered all "non-emergency US government employees" to leave Iraq, the US mission there said on Wednesday.
The embassy said it could provide only limited emergency services to US citizens. Normal visa services at US offices in Baghdad and Erbil would be suspended.
US Central Command said on Tuesday that there is an increased risk to American forces and allies from Iranian-backed militias in the region. This ran counter to an assessment by a senior British general in the US-led mission to defeat ISIS who suggested there was no increased threat from Iranian proxies.