For the first time UAE and Egyptian air forces conducted two series of joint air strikes in the past week on armed Islamist factions in Tripoli, Libya, U.S. officials said on Monday.
The officials said the two Arab countries used aircraft based in Egypt.
The New York Times earlier reported that the two U.S. allies acted without consulting Washington, and that Egyptian officials told U.S. diplomats that Cairo was not involved. Egypt has denied conducting air strikes or other military operations in Libya.
Rebel forces from the Libyan city of Misrata had already blamed the air strikes on Egypt and the United Arab Emirates, both of which have cracked down on Islamists, according to a Reuters report.
The Times quoted U.S. officials as saying Egypt had provided launching bases for the strikes, while the pilots, warplanes and aerial refueling planes were from the UAE.
State Department spokeswoman Jen Psaki refused to address the report when asked about it on Monday at a regular State Department briefing in Washington.
"I am not in a position to provide any additional information on these strikes," she was quoted as saying.
Psaki said, "Libya's challenges are political and violence will not resolve them. Our focus is on the political process there. We believe outside interference exacerbates current divisions and undermines Libya's democratic transition.
In the campaign to overthrow Gaddafi, fighters from Zintan and Misrata were comrades-in-arms. But they later fell out and this year have turned parts of Tripoli into a battlefield, with the weak government unable to control the armed factions.