The clashes came near the UN "safe area" of Bihac in the northwest.
The fighting contravened a cease-fire agreed by Bosnia's main warring parties over the New Year and prompted the United Nations to press both sides to speed up implementation of the truce.
Most of Bosnia has been quiet, but Bihac has been the scene of repeated outbreaks of fighting.
The latest clashes were fueled by Serb attempts to recapture the village of Klokot and its water reservoir, which supplies Bihac, from government Fifth Corps troops who had seized it in a surprise attack Friday.
UN observers recorded about 100 artillery explosions and scores of small-arms rounds, together with heavy machine-gun fire on the Klokot front. Fighting tailed off after dark.
Risley said the Serbs were not reported to have shelled Bihac since a weekend mortar barrage that killed seven civilians, carried out in reprisal for the Fifth Corps' attack.
But the UN was deeply concerned that the safe area could be hit again because of the fighting at Klokot, he said.
Risley said Serb forces had also ignored UN appeals to back away from the edge of another UN safe area, Srebrenica in eastern Bosnia.
The Serbs had advanced 1.5 kilometers closer to the safe area demarcation line last week under cover of the cease-fire, provoking a major clash with government troops.
The area west of the Srebrenica pocket has been generally quiet since then, but the Serb forces have not withdrawn.
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