Macedonia closed all border crossings with Kosovo yesterday after a policeman was killed not far from the frontier, a police spokesman said.
The mountain state also urged NATO to seal off Kosovo to hold back ethnic Albanian gunmen blamed for attacks that claimed more lives yesterday.
The closure cut off supplies to Kosovo, created queues of trucks inside the UN-run province and stranded dozens of international personnel working in Kosovo.
Skopje had reopened the crossings on Wednesday.
A border guard at the crossing of Blace said that, crossing into Kosovo shortly before the closure, there were fears that some armed men might be moving towards Blace.
Macedonia's Foreign Minister, Mr Srgan Kerim, called on the alliance's troops in Kosovo to clamp down on guerrillas using the province as a staging post for attacks like that on a Macedonian police convoy on Thursday that killed a serviceman.
The police remained trapped and under fire by presumed Albanian gunmen until late yesterday, when they were finally freed by reinforcements.
In neighbouring southern Serbia, a Serb policeman was killed in an ethnic Albanian guerrilla attack yesterday, the Yugoslav Interior Minister, Mr Zoran Zivkovic, told reporters. He was the latest casualty in a year of attacks by local Albanians from a buffer zone running along the border of the Albanian-dominated Kosovo province with the rest of Serbia.
Violence blamed on ethnic Albanians has also spilled into northern Macedonia in the past few weeks, rekindling international concern about regional instability.
Mr Kerim made it clear that he did not expect NATO to deploy in his own country but he did want more effective monitoring of the entire 220km (130mile) frontier to prevent extremists sowing division within stable but ethnically mixed Macedonia.
NATO has already acted to seal a small stretch of land near the Macedonian village of Tanusevci, where presumed Albanian gunmen had staged a two-week rebellion, but tighter control of the entire land border would be a mammoth job.
Mr Kerim suggested a formal buffer zone along Kosovo's border with Macedonia, an idea already rejected by NATO chiefs.
"We don't insist on forms, we insist on effective measures," Mr Kerim told reporters. "What matters is that we have effective measures on the borderline."
He went later to ask the EU for help, which responded by issuing a declaration confirming strong support for Macedonia's integrity and stability.
Macedonia has been ringing alarm bells, clearly heard in the West, for two weeks after a shadowy group which has not made any statements or identified itself on the ground occupied Tanusevci and engaged the Macedonian army in fire-fights.
On Thursday, Kfor on the Kosovo side evicted the group from Tanusevci while NATO allies decided to let Yugoslav troops into part of a buffer zone alongside Macedonia in southern Serbia.
This effectively squeezed the shadowy group from two directions.