The three-month bombardment was designed to halt Serbian strongman Slobodan Milosevic's deadly crackdown on separatist ethnic Albanians in Kosovo, a southern province that later declared independence.

The US-led campaign in effect ended a conflict that had begun in 1998 and had killed more than 13,000 people, the final chapter of Yugoslavia's bloody collapse.

Zeman, who served as prime minister between 1998 and 2002 before taking over as president in 2013, said his country was "the last" within NATO to consent to the campaign.

"I would like to use this opportunity to apologise for the bombing of former Yugoslavia," Zeman said during an official visit to Prague by Serbian President Aleksandar Vucic.

The 76-year-old Zeman, who has faced protests at home for his closeness to Russia, added that his country "desperately" sought partners within the alliance to object to the campaign, but ultimately failed.

"It was lack of courage," he said.

"With this plea of forgiveness, I resolved my long-standing trauma, because remorse is liberating.

"I said this and saved my soul."

Vucic said Serbians "will always be grateful" to Zeman.

"The words he just spoke of the bombing have never been said before," Vucic said.

The 11-week campaign was the alliance's first intervention against a sovereign state in its 50-year history.

The campaign is hailed in Kosovo as crucial in ending the war, with Westerners -- especially Americans -- enjoying unwavering popularity.

The capital Pristina even boasts a statue of Bill Clinton, who was president at the time, on a boulevard named after him.