The Obama administration is weighing several possibilities to offer military assistance to Baghdad, including drone strikes, the official told AFP on condition of anonymity.
But Baghdad has not yet formulated an official request, a US defense official said.
Faced with the Islamic State of Iraq and the Levant's spectacular assault on Mosul and seizure of a large swathe of northern and north-central Iraq, Washington is committed to "working with the Iraqi government and leaders across Iraq to support a unified approach against ISIL's continued aggression," State Department spokeswoman Jen Psaki said.
A US official said the Obama administration was considering sending "more weaponry" to Iraq after ISIL seized the cities of Mosul and Tikrit.
But there is no current plan to send US troops back into Iraq, where around 4,500 American soldiers died in the bitter conflict.
The US has already expedited arms shipments to Iraq this year and ramped up training of Iraqi security forces, while Congress is mulling a request for a further USD 1 billion in military aid.
The new USD 1 billion includes provisions for around 200 Humvee vehicles and 24 AT-6C Texan II aircraft, but it may take months to get lawmakers' approval.
Since US forces left Iraq in late 2011, Washington has provided training assistance to Iraq's military for counterterrorism missions, including in Jordan since the start of the year.
