The US was "caught off guard" by air strikes against Islamist militia in Libya, a senior official has told the BBC.
The attacks on militia positions around Tripoli airport were reportedly carried out by the United Arab Emirates (UAE) from bases in Egypt.
Egypt has denied any involvement and the UAE has not commented.
A militia alliance recently captured the capital's international airport after a battle lasting nearly a month.
The official told the BBC that the US had not been consulted about the air strikes and that it was concerned that US weapons may have been used, violating agreements under which they were sold.
The unidentified war planes attacked twice in the past week during a battle for Tripoli's airport between Islamist and nationalist militias.
A report in The New York Times on Monday said the UAE had provided the military aircraft, aerial refuelling planes and crews while Egypt gave access to its air bases.
On Monday, the US, France, Germany, Italy and the UK issued a joint statement denouncing "outside interference" in Libya which it said "exacerbates current divisions and undermines Libya's democratic transition".
WEAK POLICE AND ARMY
The BBC's Barbara Plett Usher in Washington says the air strikes have exposed another battleground in a regional struggle for power between Arab autocrats and Islamist movements.
Qatar has provided weapons and money to Islamist forces in Libya and elsewhere, she says, while Egypt and the UAE along with Saudi Arabia are trying to roll back Islamist advances.
Violence in Libya has surged recently between the rival groups who overthrew Muammar Gaddafi in the 2011 uprising.
Libya's police and army remain weak in comparison with the militias.
Over the weekend, Islamist-affiliated forces from Misrata and other cities took over Tripoli airport from the Zintan militia, which has held it for three years.
The airport, Libya's largest, has been closed for more than a month because of the fighting.
Hundreds of people have died since clashes broke out in Tripoli in July.
In another development on Monday, Libya's previous Islamist-dominated parliament reconvened and voted to disband the country's interim government.
Correspondents say it leaves Libya with two rival parliaments, each backed by armed factions.
Elections in June saw the old General National Congress (GNC), where Islamists had a strong voice, replaced by the House of Representatives, dominated by liberals and federalists.
The GNC, which reconvened in Tripoli on Monday, has refused to acknowledge the legitimacy of its successor assembly, which is based in Tobruk.
The House of Representatives says the groups now in control of Tripoli airport are "terrorist organisations".
But the Misrata-led brigade, now in control of Tripoli airport, has called on the GNC to resume work.
Libya's government has repeatedly called for the militia groups to disband and join the national army. But so far, few have shown a willingness to disarm.