They spoke as International Red Cross officials planned to evacuate thousands of Muslim and Croat civilians reportedly facing ethnic cleansing in the town.
'Over the past nine to 10 weeks, not one has passed without a report of two to three people murdered in the Banja Luka region, with other atrocities and acts of intimidation,' Kris Janowski, a spokesman for the UN High Commissioner for Refugees, said in Sarajevo.
'The Prijedor incident was the most serious such wave of violence (in this region) in a very long time.'
UN and International Red Cross envoys in north Bosnia have accepted as accurate a report by the Merhamet Muslim charity that 17 Muslims and two Croats were slain by Serbian militants in Prijedor.
Serbian police said that only seven people were killed, and asserted that the murders were not ethnically motivated but committed in the course of robberies.
UN officials who toured Prijedor, a mainly Serbian town, reported grim evidence of the attacks. They said some victims were reported to have been burnt alive in their homes, killed by grenades tossed into the houses or robbed and then summarily shot.
Mr Janowski said that the Prijedor police chief had steered him to neighbourhoods which obviously had been untouched by violence.
'The police chief insulted our intelligence by showing us areas of towns where no problems allegedly existed, where he told us how well Muslims live together with Serbs,' he said.
The police chief had agreed to assign more police to guard minority districts, and there had been no further incidents in the past 24 hours. However, the UNHCR was concerned that police were doing little to curb the attacks.
'We have the impression that the police basically turned their heads away while these things happened, staying out of the targeted neighbourhoods,' Mr Janowski said.
The UNHCR placed little credence in consistent denials by Serbian authorities of persecution of Muslims and Croats, most of whom were expelled as the Serbs overran north Bosnia in 1992, early in the ethnic war.
Mr Janowski said the UNHCR had been transporting an average of 60 Muslims and Croats out of the majority Serb region to safety in neighbouring Croatia every week since September, at their own request.
Tactics of ethnic cleansing have included arbitrary evictions, job dismissals, firebombings of minority-owned businesses and military draft notices to non-Serbs.
In east Bosnia, Bosnian Serb forces appear to be waging a concerted offensive to take the Muslim enclave of Gorazde, a UN-declared 'safe haven', by seizing a key supply road from the west.
Major Rob Annink, spokesman for the UN Protection Force, Unprofor, said yesterday that the Serbs were trying to 'finish off Gorazde with what seems to be a big-scale offensive'. Asked how the UN could tolerate the Serbs seizing an enclave that it had declared a protected zone almost a year ago, Maj Annink said: 'The members of the Security Council should object to these happenings in Gorazde. But Unprofor cannot do anything at this stage.'
UN sources said that the planned deployment of 800 Ukrainian peace-keepers in Gorazde later this month would help to deter a Serbian advance. But in the meantime Unprofor forces were fully occupied holding down ceasefires in central Bosnia.
The reports of ethnic cleansing brought condemnation from Washington, which called for those guilty of war crimes to be brought quickly to justice.