Islamic State group jihadists are making a "last stand" in their former Libyan stronghold of Sirte, where they now control only around two blocks, the Pentagon said.
The IS group had held all of the Mediterranean port city as recently as early this summer, establishing a significant foothold in Libya.
The United States started a bombing campaign in August at the request the UN-supported Libyan Government of National Accord (GNA) to help local forces recapture the city more than a year after the IS group seized it.
Although the operation has taken months longer than initially expected, it has pushed back the group's control to around 50 buildings.
The remaining jihadists are few in number "but they are persistent and fighting to the death," Pentagon spokesman Navy Captain Jeff Davis said. "It's a stubborn area."
"This is ISIL's last stand in Sirte and they are fighting hard," he added, using another acronym for the IS group.
The fall of Sirte -- the hometown of the slain dictator Moamer Kadhafi located 450 kilometers (280 miles) east of Tripoli -- would represent a significant blow to the jihadists, who have also faced a series of setbacks and major assaults in Syria and Iraq.
US warplanes, drones and helicopters have conducted 467 strikes since the air operation began on August 1.
The amphibious assault ship USS Wasp and its contingent of aircraft, which were involved in earlier strikes, left the region in October and continuing strikes are being conducted by drones.