Turkey says warlord Khalifa Haftar’s eastern Libyan forces will become a “legitimate target” if they do not immediately release six Turks.
The Turkish foreign ministry said the detention of its citizens “amounts to banditry and piracy”.
Gen Haftar’s forces said on Friday that they would attack Turkish targets over Turkish support for the internationally recognised Libyan government.
Gen Haftar started an offensive against the Tripoli government in April.
Turkey supports the Libyan government, supplying drones, weapons and trucks to boost its efforts in the ongoing conflict with Haftar’s forces.
Gen Haftar’s Libyan National Army (LNA) said it would strike Turkish vessels in Libyan waters and viewed Turkish businesses as targets.
It also banned commercial flights from Libya to Turkey.
The LNA controls most of the east and south of Libya.
Earlier on Sunday, Turkey said it would “retaliate in the most effective and strong way” to any threats from Gen Haftar’s army.
Turkey’s President Recep Tayyip Erdogan has said he is supporting the internationally recognised Government of National Accord (GNA), led by Prime Minster Fayez, to “rebalance” the fight against Gen Haftar, who has backing from UAE and Egypt.
Turkish Defence Minister Huluski Akar warned that LNA forces would pay a “very heavy price” for any attacks on Turkish interests.
On Thursday, the GNA reclaimed the strategic town of Gharyan, a main supply base for Gen Haftar’s forces in their offensive on Tripoli.
Libya has been torn by violence and division since long-time ruler Muammar Gaddafi was deposed and killed in 2011.