As Bosnia's armies fought for last-minute gains before a ceasefire due to take effect at midnight, six civilians were killed and more than 30 wounded, many of them children, by a cluster bomb fired at a refugee camp near the government-held town of Tuzla. The attack appeared to be revenge for recent battlefield losses.
The UN air base at Tuzla was also shelled, provoking an unsuccessful Nato attempt to "locate and neutralise'' Bosnian Serb heavy weapons. Poor weather and darkness forced the mission to end without success, a Nato spokesman said.
Speaking at Nato's southern command headquarters in Naples, Captain Mark Van Dyke said the planes "were not able to identify their intended target and launch weapons within the established rules of engagement''.
The first Bosnian Serb shell, armed with a cluster bomb filled with shrapnel, hit the refugee centre in Zivinice, south of Tuzla, as children were playing outside in the sunshine.
A local doctor said 20 children were among more than 30 wounded, and many were in "extremely serious" condition.
Bosnian television, whose cameras arrived only minutes after the attack, showed several of the two dozen injured children awaiting treatment. They said four children and two women were killed in the attack.
Two minutes later, a shell hit the air base without causing any damage; the refugee centre and the air base were hit again in the early afternoon, but there were no further casualties.
Although Tuzla is a UN-declared "safe area", peace-keeping officials said they could not determine whether Zivinice lay within the protected zone. But the UN was "considering a range of responses".
The Croatian state news agency said Serb planes also dropped cluster bombs on villages in the Croat-held Usora River valley in northern Bosnia, causing dozens of casualties.
The attacks came as the Bosnian Army and its Croatian allies increased the pressure on front lines in north and central Bosnia, reporting gains near the Serb-held town of Doboj. Heavy fighting was reported along the line between the contested town of Otoka, close to the Croatian border, and Kljuc, which fell to the Bosnian Army last month.
The truce was due to begin at midnight - if electricity and gas supplies are restored to Sarajevo by then. The UN and aid agencies are making frantic efforts to meet the deadline.