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By Guy Delauney
BBC News, Balkans correspondent
A row over car number plates seems an unlikely reason for placing troops on alert and falling out with international partners. But Serbia and Kosovo are currently doing precisely that.
Kosovo's insistence that ethnic-Serb residents must surrender their Serbian-issued licence plates is straining relations with its main international supporters, the EU and the United States.
Nato's Kfor peacekeeping troops are increasing patrols in case of any incidents. And Serbia's defence minister says its troops are standing by "to protect all the citizens of Serbia, including the Serbs in Kosovo".
Soon after midnight on Tuesday, Kosovo police began handing out their first written reprimands, as the deadline for the number-plate swap expired. The first recipient was a woman arriving from Serbia through the checkpoint at Jarinje, in the north of Kosovo.
The police notice warned that she must remove the Serbian-issued number plate from her car by 21 November, or face a fine. But the driver demurred, saying she would wait to hear from Serbia's government before making any changes.
There seems to be little chance of Belgrade altering its stance. It says that Kosovo Serbs have every right to use Serbian-issued number plates, reflecting Serbia's non-recognition of Kosovo's unilateral declaration of independence in 2008.
Petar Petkovic - the head of the government's office for Kosovo - sifted through the piles of documents stacked on his desk in the Palace of Serbia. He highlighted clauses which he insisted were proof that the authorities in Kosovo's capital Pristina have reneged on agreements reached through an EU-mediated dialogue.
"Pristina is the sole culprit here," he told the BBC.
"Not only is this an act of violation against the agreements concluded within the dialogue, but we now have Pristina announcing the use of violence to implement this unreasonable decision."
The allegation of "violence" refers to reports that police in Kosovo would confiscate vehicles with Serbian licence plates. In fact, the current plan is a little less dramatic.
After an initial three weeks of issuing written warnings, police will start handing out fines of €150 (£129). The next phase involves confiscating number plates - with Pristina insisting that re-registrations are complete by April next year.
Kosovo's Deputy Prime Minister, Besnik Beslimi, was due to explain all this at the Belgrade Security Conference last Friday. But Serbian police turned him back, ostensibly for his own safety, before he reached the event.
He eventually appeared via video link - and was unrepentant that Kosovo had ignored pleas from the EU and US to delay the crackdown. Pristina was simply asking for its laws to be respected, he told the BBC.
"In the past, the EU had the tendency to solve issues by postponing them," he said.
"Simply postponing only makes the problem larger - and only means that we transfer the problem to the next bureaucrat in the EU. We don't think this is the best medicine for dealing with long-term problems."
Diplomats have not bothered to hide their consternation. The EU said it was "disappointing" that its advice to delay the crackdown for 10 months "has not been respected".
Meanwhile, the US Ambassador to Serbia, Christopher Hill, expressed his exasperation when speaking during the same Belgrade Security Conference session as Besnik Beslimi.
"We've been talking about nuclear weapons lately," he said. "It's more important than licence plates."
Later, Mr Hill told the BBC that focusing on minutiae would not solve the underlying issues between Kosovo and Serbia.
"We would have liked to have seen this whole licence-plate thing shifted off so we can get on with more important issues. If there's some effort to deal with the broader issues, then issues like licence plates will fall into place."
In the meantime, residents in North Mitrovica, Kosovo's largest majority ethnic-Serb town, are caught in the middle of this latest row.
Jovana Radosavljevic lives there, running New Social Initiative - an organisation promoting trust among communities. She says many ethnic Serbs are fed up of feeling like pawns in disputes between Belgrade and Pristina.
"Over time and especially in recent history, the Serbian community in Kosovo didn't have any agency - any decision-making capacity," she says.
"Even with the Brussels dialogue and the implementation of integration, the local community was not asked how they feel, what they want and how they want it to be done."
If nothing else, the number plate dispute is serving to highlight that normalisation efforts between Kosovo and Serbia have stalled. The proverbial jump-start is sorely needed.