The United Arab Emirates has secretly carried out air strikes against militias in Libya using bases in Egypt, according to US officials.
The United States did not take part or provide any assistance in the bombing raids, two US officials said on Monday.
"The UAE carried out those strikes," one of the officials told AFP news agency on condition of anonymity, confirming a report in The New York Times.
The first air strikes took place a week ago, focusing on targets in Tripoli held by the militias, according to the US-based paper.
A second round was conducted south of the city early on Saturday targeting rocket launchers, military vehicles and a warehouse, according to the newspaper.
Those strikes may have represented a bid to prevent the imminent capture of the Tripoli airport, but the militia forces later prevailed and seized control of it.
The UAE provided the military aircraft, aerial refuelling planes and aviation crews to bomb Libya, while Cairo offered access to its air bases, the paper said.
It remained unclear whether and to what degree Egypt and the UAE had informed the Americans in advance of the air strikes.
Egypt has denied conducting air strikes or other military operations in Libya.
The new developments marked a dramatic expansion of the conflict and came as the US and its European allies denounced the "escalation of fighting and violence" in Libya and urged a democratic, peaceful transition.
A joint statement signed by Washington, Paris, Berlin, Rome and London said that "outside interference in Libya exacerbates current divisions and undermines Libya's democratic transition."
Over the weekend, Tripoli residents said unidentified war planes attacked targets in the capital, as Libya is driven by the worst fighting since the overthrow of Muammar Gaddafi in 2011.
There were also strikes on rebel-held positions last Monday.
Libya's ambassador to the United Nations, Ibrahim Dabbashi, was sceptical about Egypt and UAE involvement.
"I don't believe it," he told the Reuters news agency in New York.
"They are not even technically capable, and it would also be a very sensitive thing for them politically," he said. He declined to speculate on who else might have been behind the air strikes.