According to the US military's Africa Command, US pilots conducted 20 air strikes on Sunday alone, most of them against "enemy fighting positions."
When the Pentagon announced its latest front in the war against the IS group on August 1, officials said the campaign to help local forces push the jihadists from the coastal city of Sirte would likely be quick, taking "weeks, not months."
The military action followed a request by the UN-supported Libyan Government of National Accord (GNA), and President Barack Obama's administration has stressed that ongoing US involvement would be framed by the interim Libyan government's needs.
Navy Captain Jeff Davis, a Pentagon spokesman, said the timeline for the US air campaign continues to be dictated by the GNA.
"Every one of the strikes we do is based on a request from them, and we are down now into the last, densest part of the city," Davis said.
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"As they get to the dense areas, it's very hard to take out these sniper positions with anything other than air strikes."
The Tripoli-based GNA launched an operation in May to retake Sirte, the hometown of slain dictator Moamer Kadhafi that the jihadists have controlled since June 2015.
Since the US air operation began August 1, US warplanes, drones and choppers have conducted more than 200 strikes.
Many of the strikes are being conducted from the USS Wasp, an amphibious assault ship off the Libyan coast in the Mediterranean.
The vessel can launch Marine Corps AH-1W SuperCobra attack helicopters and Harrier jets.
The fighting has left more than 450 GNA fighters dead and 2,500 wounded. The IS death toll is not known.
The fall of Sirte, 450 kilometers (280 miles) east of Tripoli, would represent a significant blow to IS, which has also faced a series of setbacks in Syria and Iraq.