"We believe Egypt and the United Arab Emirates carried out air strikes in Libya", Pentagon spokesman, Rear Admiral John Kirby said yesterday.
The Anadolu news agency reported Kirby saying: "We discourage other nations from taking a part in Libya's issues through violence. We want the issues solved in Libya to be done peacefully and through good governance and politics and not violence."
Kirby said US Secretary of Defence Chuck Hagel "did not contact either of them [Egypt or the UAE]".
The New York Times quoted four senior US officials, which the newspaper did not name, on Monday as saying that Egypt and the UAE have "secretly cooperated to launch air strikes against Islamist militants which are battling to control Tripoli, western Libya, twice during the past seven days".
Meanwhile, Cairo and Abu Dhabi have officially denied claims that their warplanes took part in the bombing of military sites in the Libyan capital, Tripoli.
Several Libyan cities including Tripoli and Benghazi, the largest city in the east of the country have witnessed fierce clashes, the worst since the fall of former President Colonel Muammar Gaddafi in 2011. The unrest has led to the deaths of more than 200 people and forced most Western governments to withdraw their diplomatic missions from Libya.
State Department spokeswoman Jen Psaki said: "We believe outside interference in Libya exacerbates current divisions and undermines Libya's democratic transition. Obviously, that's part of our concern here, given the fact that Libya is in a very fragile place."
Psaki yesterday refused, during a press briefing, the comparison between what is happening in Libya and the US intervention in Iraq, saying: "Iraq has invited the United States in to help address the threat from ISIL. We have undertaken a range of strikes, as you know, and we have a broad, comprehensive strategy. But I would say that is one significant difference."
For the first time since the withdrawal of US troops from Iraq in January 2011, the US Air Force launched air strikes against Islamic State sites in Iraq in an attempt to stop the organisation's progress after it took control of large areas in the north and east of the country.