Turkey sent between 3,500 and 3,800 Syrian mercenaries to Libya over the first three months of the year, the US Defence Department's inspector general concluded in a new report.
The quarterly report on counter-terrorism operations in Africa by the Pentagon's internal watchdog, published on Thursday, is its first to detail Turkish involvement in Libya's war.
It says Turkey paid and offered citizenship to thousands of mercenaries fighting alongside Tripoli-based militias against troops of the Libyan National Army led by Field Marshal Khalifa Haftar.
Despite widespread reports of the fighters' extremist links, the report says the US military found no evidence to suggest the mercenaries were affiliated with ISIS or Al Qaeda.
It says they were "very likely" motivated by generous financial packages rather than ideology or politics.
The report covers only the first quarter of the year, until the end of March, two months before Turkish-backed victories by the Tripoli forces drove Field Marshal Haftar's army from the capital's suburbs, its stronghold at Tarhuna and a key western airbase.
The latest report says the Turkish troops were probably increased before the Tripoli forces' triumphs in late May.
It quotes the US Africa Command as saying 300 Turkish-supported Syrian rebels landed in Libya in early April.
Turkey also sent an "unknown number" of Turkish soldiers during the first months of the year, the inspector general said.
To the consternation of regional rivals and Nato allies such as France, Turkey is staking its hopes for greater influence in the eastern Mediterranean on the government in Tripoli.
The warring sides are stationed around the edges of Sirte, a strategic gateway to Libya's central and eastern oil crescent, where most of the country's production of 1.2 million barrels a day is located.
Egypt, which shares a porous desert border with Libya, has threatened military intervention if the Turkish-backed forces try to seize Sirte.
On Thursday, Egyptian President Abdel Fattah El Sisi hosted dozens of tribal leaders loyal to Field Marshal Haftar in Cairo, where he repeated that Egypt would "not stand idly by in the face of moves that pose a direct threat to security".
Military tension increased further this week after the collapse of a deal to end the blockade of Libyan oilfields, which has deprived the country of its most important economic resource and the National Oil Corporation of more than $7 billion (Dh25.71bn) in revenue.
On Friday, the National Oil Corporation warned that international powers were pulling the country towards an escalation that would probably extend to the oil and gas facilities.