Two Serbian nationals held hostage in Libya since November were among those killed in Friday’s air strikes by US warplanes, the Serbian government says.
The jets targeted positions of the self-styled Islamic State group (IS) in the city of Sabratha.
The kidnapped Serbs, Sladjana Stankovic and Jovica Stepic, worked at the Serbian embassy in Libya.
Prime Minister Aleksandar Vucic described their deaths as “terrible collateral damage”.
Earlier, Foreign Minister Ivica Dacic told a news conference the Serbian authorities had been trying to negotiate their release but the kidnappers’ financial demands had been “impossible” to meet.
Davic said a protest note would be sent to Washington for failing to inform the Serbian authorities before the raid.
Ms Stankovic, a communications officer, and Mr Stepic, a driver, were seized on 8 November after their diplomatic convoy, bound for Tunisia, was rammed and fired on near Sabratha. Serbian ambassador Oliver Potezica escaped unharmed.