The official said that Washington had not been consulted about those air strikes and was caught off guard.
The strikes on positions held by the militants around the international airport in Tripoli were carried out by Emirati jets that used bases in Egypt.
Authorities in Egypt denied any involvement, and no direct comment has been made by the UAE.
The strikes did not stop the militants from Misrata as well as other cities, which are in a group called Libya Dawn, from seizing the airport, which had been under control since 2011 by a militia from Zintan.
Sources in Libya said the Misrata alliance was now in control of Tripoli since rivals withdrew from the capital.
Life there has drawn to a standstill as many offices and businesses are closed, while residents sit in fear of what will happen next.
Many people have died during over a month of fighting and the airport is in ruins.
U.S. officials reporting of the air strikes around the airport in Tripoli are remarkable for many reasons.
First, a small state in the Gulf, staging from an Egyptian airfield, carried the attack out.
This shows a growing unease amongst the traditional rulers of the region because of the upheavals the Islamist radicals have prompted.
Secondly, neither Egypt nor the UAE mentioned the airstrikes beforehand to Washington. That is a measure of the fall in the standing of the White House in the region. The Obama administration is seen by many there as vacillating and hesitant in its decision-making.
Of course, the United States were involved significantly in ousting the former regime in Libya but since then it nor France and Britain have shown an ability to influence ground events.
The official from the U.S. said that they were not consulted and are concerned that the weapons used could have been from the U.S., which violates the agreement under which they had been sold to the UAE.