Kosovo Prime Minister Albin Kurti has called for the opening of a flashpoint bridge, prompting protests, and authorities this week raided post offices long used by residents who maintain funds in Serbia.

"We are concerned by continuing uncoordinated decisions by the leadership of Kosovo," State Department spokesman Matthew Miller said.

Noting longstanding US support for Kosovo's integration into the international community, Miller told reporters: "We believe they put at risk the opportunities that we have helped Kosovo create."

"What we would encourage the government of Kosovo to do is to return to close and constructive engagement with the United States, with the EU, with NATO."

The United States led a NATO bombing campaign in 1999 that brought self-rule to Albanian-majority Kosovo from Serbia after a war that left 13,000 dead.

Kosovo declared independence in 2008, a move never acknowledged by Serbia but recognized by most, although not all, Western nations including the United States.

A series of recent steps by Kurti's government have increased tensions, including making the euro the only legal currency in Kosovo, effectively outlawing use of the Serbian dinar.

The NATO-led peacekeeping force KFOR warned Tuesday it would intervene if Kosovo sought to reopen the bridge separating communities in divided Mitrovica, the scene of frequent clashes.