Biden landed on board US Air Force Two plane at Sarajevo airport at dawn under heavy security measures.

He was spending Tuesday in Bosnia-Hercegovina -- accompanied there by the European Union's foreign policy chief Javier Solana -- meeting with political leaders and delivering a speech in the Bosnian parliament.

Biden, the highest-level US official to visit Bosnia since President Bill Clinton in 1999, is expected to call on rival ethnic leaders to enact reforms critical to Bosnia's hopes of joining the EU and NATO military alliance.

Coming only four months after Barack Obama was sworn in as US president, Biden's three-day trip will take in Bosnia, Serbia and Kosovo.

It follows the adoption in the US House of Representatives last week of a resolution calling for urgent constitutional reforms in Bosnia and the appointment of a US special envoy to the Balkans.

Hailing the visit as "historic," Sarajevo's Dnevni Avaz newspaper said Biden's trip means the "Balkans have again been put among the priorities of US foreign policy."

But Biden was also expected to warn Bosnia's two main political figures -- Bosnian Serb Prime Minister Milorad Dodik and the Muslim member of Bosnia's tripartite presidency, Haris Silajdzic -- against their use of inflammatory nationalist rhetoric.

Bosnia's political scene has been tense since 2006 elections propelled into office the two leaders.

Dodik has warned Republika Srpska could secede from Bosnia, while Silajdzic has called for the semi-autonomous Serb entity to be abolished. Their frequent public clashes have been blamed for stalled EU-backed reforms.

The Clinton administration was the main advocate of the 1995 NATO bombing of Serb positions in Bosnia.

The air strikes forced Serbs to end the war through a peace deal reached in Dayton, Ohio. The accord split Bosnia into two highly independent entities -- the Serbs' Republika Srpska and the Muslim-Croat Federation.

Serb veterans of Bosnia's 1992-1995 war which claimed 100,000 lives staged several small protests against the visit in their entity.

"By lighting candles, we are making a peaceful protest against the fact that there is a high level of discrimination for non-Muslims in Bosnia," protest organiser Pantelija Curguz told journalists in Banja Luka.

At one demonstration in Pale, Serbs' wartime stronghold near Sarajevo, some protestors carried photographs of their former leader Radovan Karadzic, who is facing genocide charges before a UN court in The Hague.

On Wednesday, Biden is to travel to Belgrade, whose long-strained ties with Washington worsened when the US backed Kosovo's unilateral declaration of independence from Serbia last year.

"The objective in a sense is very straightforward," a US administration official told reporters in Washington ahead of Biden's trip.

"This is a tremendous opportunity to make it very clear to the government of Serbia and to the people of Serbia that we hope to press the reset button with Serbia," the official said on condition of anonymity.

Serbian authorities have beefed up security ahead of the visit, including a ban on public demonstrations.

On Thursday, Biden travels to Kosovo, where he is expected to urge the ethnic Albanian majority's leaders to build a functioning and effective state and to protect the rights of minorities, especially Serbs.

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