Iraqi forces backed Popular Mobilisation Forces have opened supply lines into the town of Beiji and its nearby oil refinery.
The development came as the UN condemned abuses of the so-called Islamic State of Iraq and the Levant (ISIL) group against women.
Colonel Steve Warren, the defence department spokesman, said the US was not ready to declare Beiji refinery, the country's biggest, or the town "liberated", but progress was being made by Iraqi troops.
"Friendly forces have begun moving into the town ... and are methodically beginning to root out the enemy," Warren said.
At the refinery, Iraqi troops broke through to a contingent of Iraqi troops dug in at the northwest corner of the facility for several months and were able to deliver equipment and personnel to the men on the ground.
"This was a methodical and often times grinding movement from the south to the north," Warren said.
"It took the friendly forces several weeks to be able to break through" an ISIL defensive line, the Pentagon spokesperson was quoted by Reuters news agency as saying.
The Iraqi force is comprised of a mix of conventional troops and the Popular Mobilisation Forces who answered a call by political and religious leaders to take on ISIL after its advances last year.
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