Tensions are flaring in northern Kosovo, where ethnic Serbs are blockading roads leading to two border crossings with Serbia.

Hundreds of ethnic Serbs protested at the sealed crossings at Jarinje and Brnjak Friday, reacting to a new attempt by European Union police and Kosovo customs officials to re-open them to traffic.

In Pristina, Kosovo Prime Minister Hashim Thaci told Cabinet ministers efforts to reopen the customs posts are under way, but Serbia's minister for Kosovo, Goran Bogdanovic, warned Friday the effort could lead to “renewed conflict.”

EU officials say they are just trying to implement an agreement made earlier this month to reopen the crossings.

The EU said helicopters are being used to ferry personnel and supplies to the posts, which are surrounded by barbed wire. And a statement by EULEX, the EU mission in Kosovo, called on both sides to support the measures, “to improve the rule of law for everyone.”

In Washington, U.S. State Department spokesman Mark Toner called on all parties to maintain calm, saying roadblocks and barricades only serve to impair the daily lives of the people of both Kosovo and Serbia. He also commended the efforts of NATO and EULEX to ensure security in the region and enable the free movement of both goods and people over the border.

The crossings have been under NATO control since violence between ethnic Serbs and Albanians erupted there in July, leaving one person dead.

A senior United Nations peacekeeping official urged Kosovo and Serbia Thursday to demonstrate their commitment to a peaceful resolution ahead of the planned reopening of the border crossings.

U.N. Assistant Secretary General for Peacekeeping Operations Edmond Mulet also said Pristina and Belgrade need “to take responsibility” for preventing any recurrence of violence.

Serbia and Russia had requested the U.N. Security Council meeting to help prevent Kosovo's ethnic Albanian authorities from using force in northern Kosovo, which is populated by ethnic Serbs who reject the government in Pristina.

Kosovo declared independence from Serbia in 2008, and many countries, including the United States, have recognized the move. But Serbia claims the move is illegal and has the support of Russia, a permanent U.N. Security Council member with veto power. Since then, northern Kosovo has been plagued by ethnic conflict between the region's majority Serbs and ethnic Albanians, who are the majority in the rest of Kosovo.

Serbian customs officials have barred goods that contain attributes of Kosovo's statehood, such as Pristina's own stamps. In response, Kosovo has barred Serbian imports and attempted to take over the two important border crossings to enforce the ban.