BELGRADE (Reuters) - The United States urged Belgrade and Pristina on Tuesday to avoid “dangerous rhetoric” after Kosovo denied entry to a train painted in the Serbian national colors with “Kosovo is Serbia” emblazoned on its side.

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Kosovo’s President Hashim Thaci has accused Serbia of planning to seize a slice of northern Kosovo using the “Crimea model,” a reference to the Russian annexation of the peninsula.

His Serbian counterpart Tomislav Nikolic has said Pristina showed it wanted war by deploying special police at the border to block the train. The train was supposed to travel to ethnic Serbian enclaves in Kosovo.

Kosovo, backed by the U.S. and major west European states, declared independence from Serbia in 2008. Belgrade considers it part of its territory and supports a Serb minority there.

“Kosovo is a sovereign, independent country and we respect the right of Kosovo to manage who and what crosses its borders,” the U.S. embassy in Pristina said in a statement.

“We urge all sides to avoid dangerous rhetoric and continue to work for the normalization of relations.”

Russian Foreign Minister Sergei Lavrov said on Tuesday tensions were sharply rising in the Balkans and the European Union must help de-escalate the situation there.

NATO air strikes on Serbia forced it to withdraw its troops in 1999 after killing about 10,000 ethnic Albanian civilians there. NATO still has around 5,000 troops stationed in Kosovo to keep the fragile peace.

Nikolic said after meeting the U.S. ambassador to Belgrade on Tuesday that U.S. politics had caused “trouble” in the region and expressed hoped the new Trump administration would be more supportive of Serbia’s policies.

Relations between Belgrade and Pristina came under renewed strain on Jan. 4 when former Kosovo prime minister Ramush Haradinaj was arrested in France on a warrant from Serbia, which accuses him of war crimes.

Kosovo Albanians make up more than 90 percent of Kosovo’s 1.8 million population. Northern Kosovo is home to a Serb minority of around 40,000 to 50,000 people who do not consider Pristina as their capital.

Normalizing relations between Kosovo and Serbia is key condition for both countries to progress toward membership in the European Union both governments are aiming for.