NATO's KFOR peacekeeping force says the situation in northern Kosovo is tense but they are "prepared to intervene" if it become unstable.
The Kosovo government has delayed a decision to oblige Serbs living in the country's northern municipalities to apply for Pristina-issued number plates following a set of fresh tensions between local communities and law enforcement officials.
Late on Sunday evening, protesters parked trucks filled with gravel and other heavy machinery on roads leading to the two border crossings, Jarinje and Bernjak, in a territory where Serbs form a majority. Kosovo police said they had to close the border crossings.
Police also reported shots being fired in their direction earlier on in the day, but without any injuries. Protesters are also believed to have attacked several Albanians and cars passing on the roads that had been blocked.
Air raid sirens were heard for more than three hours in the small town of North Mitrovica inhabited mainly by Serbs.
Approximately 50,000 Serbs living in the north still use number plates and documents issued by Serbian authorities, despite Kosovo's declaration of independence from Serbia in 2008. The nation is recognised as an independent state by more than 100 countries, but not by Serbia or Russia.
"The overall security situation in the Northern municipalities of Kosovo is tense," NATO-led mission to Kosovo KFOR said in a statement. It further stated that it would be "prepared to intervene" if the instability were to grow worse.
In Moscow, Russian Foreign Ministry spokeswoman Maria Zakharova blamed the heightened tension on what she called "groundless discriminatory rules" imposed by Kosovo authorities.
As a result of such rising tensions, the Kosovo government - led by prime minister Albin Kurti - said it would grant Serbs a 60-day transitional period to get Kosovo number plates, a year after giving up trying to impose them due to similar protests.
The government also decided that as of 1 August, all citizens from Serbia visiting Kosovo would have to get an extra document at the border to grant them permission to enter - a rule similar to that applied by Serbia to Kosovar visitors.
But following tensions on Sunday evening and consultations with EU and US ambassadors, the government said it would delay its plan for one month, and start implementation on 1 September.
A year ago, after local Serbs blocked the same roads over number plates, Kosovo's government deployed special police forces and Belgrade flew fighter jets close to the border.
Tensions between Serbia and Kosovo remain high and the latter's fragile peace is maintained by a NATO mission which has 3,770 troops on the ground. Italian peacekeepers were visible in and around Mitrovica on Sunday.
The two countries committed in 2013 to a dialogue sponsored by the European Union to try to resolve outstanding issues, but with little progress so far.