Senior U.S. officials said Egypt and the United Arab Emirates have launched airstrikes against Islamic militants in the Libyan capital twice in the past week.
U.S. officials told reporters that the move caught the United States by surprise, potentially dealing a blow to relations between Washington, Cairo and the Emirates.
U.S. officials said Egypt provided the base for the launch of the airstrikes, and the U.A.E. provided the aircraft and pilots.
Egypt has not publicly acknowledged any role in the air strikes. The UAE had no immediate comment.
The first strike took place in Tripoli a week ago and targeted Islamist-alligned militant-held facilities, such as a weapons depot, and killed six people, according to The New York Times.
The second airstrike hit militant-controlled rocket launchers, military vehicles and a warehouse in Tripoli, killing a dozen people.
U.S. officials, according to the Times, say the strikes have, so far, proved counterproductive. The militants gained control of the Tripoli airport just hours after the second attack.
Warning against interference
On Monday, the United States and four top European allies warned against outside interference in Libya.
The State Department, along with Britain, France, Germany and Italy, issued a statement saying actions by outsiders exacerbate divisions in Libya and undermine democracy.
The five allies strongly condemned ongoing fighting in such major cities as Tripoli and Benghazi, especially in residential areas, and urged all parties to accept a cease-fire.
Libya has been in turmoil since the 2011 ouster of dictator Moammar Gadhafi.
Numerous armed Islamist groups are violently jockeying for power, competing assemblies have elected rival prime ministers, and an interim central government is struggling to establish security.
The violence in Tripoli and Benghazi has prompted several countries to evacuate their citizens and diplomats from Libya.
Officials from Libya's neighbors - Algeria, Chad, Egypt, Sudan and Tunisia - have met in Cairo to discuss the worsening chaos.
Egyptian Foreign Minister Sameh Shukri said the weapons stockpiled by all the Libyan militias should be confiscated. He has voiced concern about the effect of the turmoil on Libya's neighbors.