BENGHAZI, Libya, Sept 12 (Reuters) - A squad of U.S. troops despatched by helicopter across the Libyan desert to rescue besieged diplomats from Benghazi on Wednesday ran into a fierce overnight ambush that left a further two Americans dead, Libyan officials told Reuters.

Accounts of the mayhem at the U.S. consulate, where the ambassador and a fourth American died after a chaotic protest over a film insulting to Islam, remain patchy. But two Libyan officials, including the commander of a security force which escorted the U.S. rescuers, said a later assault on a supposedly safe refuge for the diplomats appeared professionally executed.

Miscommunication which understated the number of American survivors awaiting rescue - there were 37, nearly four times as many as the Libyan commander expected - also meant survivors and rescuers found themselves short of transport to escape this second battle, delaying an eventual dawn break for the airport.

Captain Fathi al-Obeidi, whose special operations unit was ordered by Libya's authorities to meet an eight-man force at Benghazi airport, said that after his men and the U.S. squad had found the American survivors who had evacuated the blazing consulate, the ostensibly secret location in an isolated villa came under an intense and highly accurate mortar barrage.

"I really believe that this attack was planned," he said, adding to suggestions by other Libyan officials that at least some of the hostility towards the Americans was the work of experienced combatants. "The accuracy with which the mortars hit us was too good for any regular revolutionaries."