Biden, an advocate of NATO bombing in 1999 that stopped Serbia's crackdown on ethnic Albanian separatists, offered condolences to families of those killed in the 1990s as he visited Belgrade on Tuesday.

"The memories of the loss of loved ones are still fresh," Biden told reporters before taking a walk with Serbian Prime Minister Aleksandar Vucic in the Serbian capital where the battered ex-defense ministry buildings still exhibit damage.

Biden traveled late Tuesday on to Pristina, the capital of Kosovo, where ethnic Albanian rebels sought independence after the US-led NATO bombing campaign. Kosovars still regard Washington as their rescuer.

Kosovo declared independence in 2008 and has since been recognized by more than 110 countries. Serbia does not recognize Kosovo sovereignty.

Some 10,000 people died and 1,700 went missing during the 1998-1999 war as Kosovo fought to break away from Serbia.

T-shirts for Trump

As Biden visited Belgrade, some 300 ultra-nationalists protested his presence by wearing T-shirts displaying Republican US presidential candidate Donald Trump and chanting "Vote for Trump!"

Biden, a Democrat, said Serbia, being the most populous nation in the western Balkans, was crucial for peace in the Balkans - what was once Yugoslavia and now comprises Croatia, Slovenia, Montenegro, Bosnia Herzegovina and Macedonia - alongside Albania.

He stressed the importance of ongoing EU-brokered talks to normalize relations between Kosovo and Serbia, which since 2008 has sought EU membership.

Vucic, an ultra-nationalist turned pro-EU reformer, warned that "only a spark" could turn simmering tensions into "chaos."

Belated EU membership advocate Vucic

Among the ultra-nationalists, Radical Party leader Vojislav Seselj described Trump as a "friend" of the Serbian people and Russia.

Counteracting Russia's sway

Associated Press said Biden's visit demonstrated Washington's desire to counteract Moscow's historical sway over Serbia and maintain US influence.

His trip also highlighted Washington's concern about the slow pace of regional reconciliation 17 years after the NATO airstrikes and 21 years after the Dayton peace deal for Bosnia that left it as a set of regional "entities."

Victims of Bosnia's 1992-95 war reacted with dismay in March when Seselj, notorious for his ethnic rhetoric, was acquitted by the UN tribunal in The Hague.

ipj/kl (Reuters, AP, AFP)