This story was originally published Aug. 1, 2016, at 11:10 a.m. EST.

US warplanes on Monday launched a new round of airstrikes against Islamic State militants in Libya, a campaign that is expected to last at least the next several days, the Pentagon has confirmed.

"Today, at the request of the Libyan Government of National Accord (GNA), the United States military conducted precision airstrikes against ISIL targets in Sirte, Libya, to support GNA-affiliated forces seeking to defeat ISIL in its primary stronghold in Libya," Pentagon Press Secretary Peter Cook said in a statement, which references one of the Islamic State group's acronyms.

Sirte, located along the Libyan coast, is the scene of prior limited US airstrikes targeting a growing militant presence there. Monday's strikes were conducted by a combination of manned and unmanned aircraft, defense officials said.

Defense officials will not say whether there are any U.S. forces on the ground in Libya.

There are believed to be about 1,000 ISIS fighters in Sirte, defense officials said. The last round of airstrikes there were conducted in February. Since then, US officials have been reluctant to authorize more until a cohesive government emerged from the country's chaotic civil war.

These new developments follow months of discreet ground operations, as small teams of US special operations troops have moved in and out of Libya, making contact with rebel factions and gathering intelligence about the political and military situations there.

The attack was authorized by US President Barack Obama on the recommendation of Defense Secretary Ash Carter and Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford, Cook said.

Cook called the attack "consistent with our approach to combating ISIL by working with capable and motivated local forces. GNA-aligned forces have had success in recapturing territory from ISIL thus far around Sirte, and additional U.S. strikes will continue to target ISIL in Sirte in order to enable the GNA to make a decisive, strategic advance."

Col. Mark Cheadle, a spokesman for US Africa Command (AFRICOM), told Military Times: "We are employing a variety of platforms to provide key information to the GNA-aligned forces. As well, we have the ability to conduct manned and unmanned airstrikes against [ISIS] targets in Sirte to help enable the GNA-aligned forces to make a decisive and strategic advance."

The US amphibious assault ship Wasp, carrying an element of the 22nd Marine Expeditionary Unit, is standing by in the vicinity of Libya, sources said. That includes AV-8B Harrier attack jets. The Wasp is not accompanied, sources said, by the other two ships of its amphibious ready group.

The attack Monday was part of a comprehensive series of operations planned and controlled by AFRICOM. The first element of this three-phase plan is Operation Odyssey Resolve, consisting of intelligence, surveillance and reconnaissance flights designed to counter violent extremism in Libya.

The second phase, Operation Junction Serpent, provided targeting information. The third element, Operation Odyssey Lightning, includes strike aircraft hitting those targets. That operation reportedly began over the weekend, Pentagon sources said.

In February, U.S. intelligence officials raised their estimate for ISIS fighters in Libya to between 5,000 to 6,000, up from previous estimates of 2,000 to 3,000.