SARAJEVO -- Signs of a superpower rift over Bosnia emerged Friday when the United States contradicted Russia's view that the Serbs had responded positively to the latest international peace plan.


The Bosnian Serbs failed to give clear backing to the plan drafted by the United States, Russia, Britain, Germany and France and said it could serve as a basis for further negotiations.


Russia described this reply as "rather positive," but U.S. Defense Secretary William Perry, on a visit to former Yugoslavia, said the Serb response was disappointing and did not amount to acceptance.


In 27 months of war, the Serbs have proved adept at exploiting differences within the international community in order to strengthen their bargaining position against their Moslem and Croat foes.


The latest peace plan awards the Moslem-Croat alliance 51 percent of Bosnia, with the rest going to the Serbs, who would have to hand back nearly one third of the land they now hold.


Perry held talks in Zagreb on Friday with NATO and UN peacekeeping commanders on what military steps might be taken against the Serbs to force them to comply with the peace plan.


Afterwards, he disagreed with the Russian view of the Serbs' response to the plan at talks with the five-power "contact group" in Geneva this week.


"It was not an acceptance of the plan and that is what the contact group had asked for," Perry told reporters. "We considered it not a positive answer but a disappointing answer and it is going to greatly complicate the path ahead."


He added: "We would hope, sincerely hope, that between now and the July 30 meeting" of contact group foreign ministers "that the Bosnian Serbs will reconsider their answer and give them something more positive to work on at that meeting."


Perry had been scheduled to hold his talks in Sarajevo but was forced to divert to Zagreb after hostile fire at United Nations aircraft caused the UN-controlled airport in the Bosnian capital to be closed.


The UN said the airport remained closed to relief flights Friday, the third consecutive day the aid airlift has been suspended.


A United Nations spokesman reported a gradual increase in fighting between Serbs and Moslems on Thursday along a key supply route in north-central Bosnia.


Two people were killed and five wounded by mortar fire on the Moslem-held town of Maglaj. Fresh fighting was also reported from the northwestern enclave of Bihac, with more than 400 shells landing.


In Sarajevo, UN military spokesman Rob Annink said Thursday's shooting incidents were the worst attacks on aircraft landing and taking off in the two years the city's airport has been open under UN control.


Annink said the shots appeared to come from a place where Serb and Moslem forces were close to one another, making it impossible for the United Nations to single out either side for blame.


In Thursday's attacks, an Ilyushin 76 transport plane was hit by six rounds of small arms fire during take off.


A U.S. C-141 was shot at while landing and again on take-off, receiving a total of 30 to 40 hits.


A third plane, an Antonov 26, was also shot at on landing and take-off -- both times by machine gun. A U.S. civilian working for the United Nations was wounded in the incident.


While the Moslem-led Bosnian government accepted the peace plan, the Serbs attached a list of conditions to their response which Western diplomats said amounted to a rejection.


This prompted the Bosnian government first to withdraw its acceptance of the deal and then reverse that decision following a telephone conversation between U.S. envoy Charles Redman and Bosnian President Alija Izetbegovic.


Redman told reporters from Brussels that Izetbegovic "told me very clearly their unconditional acceptance is still on the table, that it was never off the table."