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.
Mr Davic said a protest note would be sent to Washington for failing to inform the Serbian authorities before the raid.
Libya's internationally recognised government, based in the east of the country, was also angered by the US raid.
It said in a statement on Saturday that the attack had not been co-ordinated with Libyan authorities and was "a clear and flagrant violation of sovereignty of the Libyan state".
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.
The Pentagon said on Saturday it had "no information" that the Serbs' deaths were a result of the US strikes.
Pentagon spokesman Peter Cook said: "Our forces watched this training camp for weeks leading up to the operation, and at the time of the strike there were no indications of any civilians present."
The dozens killed in Friday's air strikes on the IS camp reportedly included Tunisian extremist Noureddine Chouchane.
He has been linked to two attacks in Tunisia last year, including one which killed 30 Britons in the beach resort of Sousse.
"We took this action against Sabir [Noureddine Chouchan] in the training camp after determining that both he and the Isil [IS] fighters at these facilities were planning external attacks on US and other Western interests in the region," Mr Cook said.
IS has been active in Libya for more than a year. The US estimates the group has up to 6,000 fighters there.
More than four years after the overthrow of former Libyan leader Muammar Gaddafi, the country remains fragmented, fought over by a number of groups, including IS.
Different authorities rule over various regions, hampering efforts to find a unified national administration.
Islamic State militants in Libya
- IS jihadists seized a strip of land along Libya's coast, around the city of Sirte, last year
- Libyan intelligence officials say foreign fighters and some IS commanders have relocated there from Iraq and Syria
- The US estimates the number of IS foreign jihadists at 4,000-6,000
- The largest groups are said to be from Tunisia, Egypt, Sudan and Iraq
- Recent recruitment has focused on sub-Saharan African countries
- It is not known how many of the jihadists are nationals of European countries
- IS militants have attacked Libyan cities, including Tripoli and Benghazi, and oil installations