President Trump authorized the airstrikes based on the recommendation of Defense Secretary Jim Mattis, an administration official said on Sunday.
The Africa Command statement noted that the strikes, which military officials said separately were carried out by armed Reaper drones flying from a base in Sicily, were conducted in coordination with the government of Prime Minister Fayez Serraj of Libya.
Reda Eissa, a spokesman for the coalition of militias backed by Mr. Serraj’s Government of National Accord, said he had no information about Friday’s airstrikes. He referred queries to the government’s defense ministry, which did not respond to requests for comment.
Libya analysts said the area where the strikes took place is controlled by another major power broker in Libya’s fractious political landscape, Gen. Khalifa Haftar, a former Libyan Army officer who was a major figure in the 2011 rebellion against the former Libyan dictator, Col. Muammar el-Qaddafi.
The American airstrikes come at a particularly chaotic moment for Libya. In the east, General Haftar has strengthened his position, receiving a stream of Western ministers, including the British foreign secretary, Boris Johnson.
Elsewhere, the Italian government has struck deals with smuggling militias to cut human trafficking to Europe. The capital, Tripoli, has seen sporadic violence as rival groups, including the United Nations-backed unity government, jostle for supremacy.
The American strikes took place in a desert region south of the oil crescent, where key ports changed hands between rival groups several times last spring.
“The strikes are the logical consequence to the change of the tactics of I.S.,” said Col. Wolfgang Pusztai, a former Austrian defense attaché to Libya who follows the country closely, referring to the Islamic State. “This is to keep the pressure on them, as none of the Libyans considers the fight against I.S. as a priority.”
On Wednesday, in the latest attempt to revive Libya’s stuttering peace efforts, the United Nations envoy Ghassan Salamé said he would seek to renegotiate a national political agreement signed in Morocco in December 2015. That announcement, at the United Nations in New York, acknowledged the chronic weakness of the unity government, which has dismally failed to assert its authority beyond Tripoli, yet retains the backing of the international community.
Mr. Salamé said the United Nations was ready to facilitate a “security dialogue” with Libya’s many armed factions with a view to drawing them into the political process.
American military officials said on Sunday that Friday’s strikes were conducted under Obama-era rules designed to protect civilians outside active war zones, like Iraq and Afghanistan. Those rules — called the Presidential Policy Guidance, or P.P.G. — are in the process of being replaced by the Trump administration to streamline the process for approving drone strikes and commando raids in areas outside traditional war zones, including Yemen, Somalia and Libya.
Issued by Mr. Obama in 2013, those rules require “near certainty” that a bombing will kill no civilians, and the target must pose a threat to Americans — not just to American interests. The new procedures awaiting Mr. Trump’s approval would lay the groundwork for expanding possible counterterrorism missions in countries where Islamic militants are active but the United States has not previously tried to kill or capture them.
For the more than 500 other airstrikes in Libya this year and last year, the Obama White House authorized an exemption to its rules and effectively made Libya a fourth combat zone, along with Iraq, Syria and Afghanistan. It was not immediately clear why the Trump administration did not temporarily turn back on the exemption from the Obama-era rules.