SARAJEVO -- Bosnian Serbs, testing international resolve over the latest Bosnian peace plan, closed key routes for civilian traffic into Sarajevo on Wednesday and threatened to block UN convoys.


The Serbs said they would cut off water, gas and electricity supplies to the Bosnian capital and block UN convoys through their territory if Serb prisoners were not freed by Sunday, UN officials said.


The United Nations called in NATO warlplanes after two British soldiers and a Bosnian civilian were injured after being fired at by Serbs near Sarajevo.


In the firing two UNPROFOR trucks caught fire, one carrying fuel. The NATO planes could not identify their target and so did not open fire.


UN officials reported Bosnian Serbs had closed vital roads to non-UN traffic crossing Sarajevo's airport Wednesday morning. The move was apparently aimed at increasing their leverage in peace negotiations.


Bosnian Serbs have stepped up pressure on the United Nations as major powers debate whether to take punitive measures to force Serb compliance with the peace plan, which the Serbs effectively rejected last week.


But diplomats said there were signs the Serbs were ready to cut a deal that could break the impasse over the peace plan drawn up by the five-nation contact group comprised of the United States, Russia, Britain, France and Germany.


"At least some on the Serb side seem willing to climb down from an outright rejection of the contact group plan and make a deal," a diplomat in Sarajevo, who asked not to be named, said.


But it remained uncertain if the Moslem-Croat Bosnian federation would agree to a deal that would involve land concessions and granting Serbs the right to secede from Bosnia.


The road closure revived memories of the early days of the city's siege and Sarajevans braced for a return to grim living conditions and sharp rises in prices for food and fuel.


Bosnian Serb leader Radovan Kar-adzic said the closure was temporary.


"The transit across Sarajevo airport is temporarily suspended," he told the Bosnian Serb news agency SRNA "because Moslem snipers are still killing Serbs, they failed to respect agreement on exchange of prisoners and are on a constant offensive on all battlefields in Bosnia."


UN special envoy Yasushi Akashi sent a letter to Karadzic calling for the Serbs to reopen the airport roads.


Russian Defence Minister Pavel Grachev and special envoy Vitaly Churkin held lengthy talks with Serb leaders in Belgrade on Tuesday. A Serb official said later the self-declared Bosnian Serb assembly may hold a session to reconsider the peace proposa