US officials say Egypt and the UAE were behind air strikes in Libya last week that targeted Islamist-linked militia.
A senior US official told the BBC that Washington was not consulted about the attacks and was "caught off-guard".
The air strikes on militia positions around Tripoli's international airport were reportedly carried out by Emirati fighter jets using bases in Egypt.
The Egyptian authorities have denied involvement, and there has been no direct comment from the UAE.
The strikes failed to stop militias from Misrata and other cities, which operate under the banner Libya Dawn and include some Islamist groups, seizing the airport from a militia from Zintan that had controlled it since 2011.
The BBC's Rana Jawad in Tripoli says the Misrata alliance is now effectively in control of the capital because their rivals have withdrawn from the city.
She says that life has largely come to a standstill, with many business and offices closed, while many residents are afraid.
Hundreds of people have died during more than a month of fighting which has left the airport in ruins.
Analysis: Jonathan Marcus, BBC News
What US officials have described as a pair of air strikes in and around Tripoli airport by jets from the United Arab Emirate is remarkable for a number of reasons.
Firstly the fact that the small Gulf state - staging from an airfield in Egypt - carried out the attacks at all.
This is very unusual - a mark of the growing unease of the region's traditional rulers at the upheavals prompted by radical Islamists.
Secondly there is the fact that neither the UAE, nor Egypt, told Washington in advance. It is a measure of the declining standing in the region of the Obama administration, which is widely seen as hesitant and vacillating.
Of course, the US was significantly involved in the operation to oust Libya's former regime, but since then, neither it nor its partners like Britain and France, have shown much ability to influence events on the ground.
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.
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.
The first air strikes, which took place on 18 August, targeted a small weapons depot and other militia positions, US officials told the newspaper. Six people were reportedly killed.
Then on Saturday, jets bombed rocket launchers, military vehicles and a warehouse controlled by militiamen, the officials added, leaving at least a dozen people dead.
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".
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 militia.
Battle for Libya:
Misrata-led alliance (Libya Dawn):
- Effectively in control of Tripoli
- Includes some Islamist militias, such as LROR
- Also Libya Central Shield
- Called on former parliament, GNC, to reconvene
- Had controlled Tripoli airport for three years
- Includes al-Qaqa Brigade and al-Sawaiq Brigade
- Al-Qaqa had officially joined the national army but remained autonomous
- Supports new parliament
- Loosely allied with rogue General Haftar, who has been attacking Islamist groups in eastern city of Benghazi
In another development on Monday, some members of Libya's previous Islamist-dominated parliament reconvened and voted to disband the country's interim government.
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 has refused to acknowledge the legitimacy of its successor assembly, which is based in Tobruk and is recognised by the international community.
The GNC's mandate expired in June and not enough of its former members gathered in Tripoli on Monday to form a quorum.
The House of Representatives says the groups now in control of Tripoli are "terrorist organisations".
But the Misrata-led brigade 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.