Artillery and mortar barrages killed 12 people in Sarajevo on Saturday, including three who died while waiting in line for humanitarian aid.
The shelling intensified as Britain announced an international conference in August to try to restore peace in Yugoslavia after a year of conflict between its former republics.
Radio journalists in the Bosnian capital said explosions could be heard across the city early in the evening as Muslim and Croat forces traded shells with their Serbian besiegers.
Fighting also broke out in the mainly Muslim suburb of Dobrinja after a brief cease-fire to permit the distribution of food.
Also Saturday, a relief convoy straggled back to Sarajevo after failing to reach the besieged city of Gorazde.
"The road to Gorazde is not safe. A horrible, terrible, incredibly stupid war is going on there," Fabrizio Hochschild, director of relief operations in Sarajevo for the U.N. High Commissioner for Refugees, said after mines and bullets turned back his convoy.
Serbian media from Belgrade reported that the convoy was attacked by Bosnian Muslims. Mohammed Zlatan Hrenovica, a spokesman for the Bosnian Defense Forces in Sarajevo, blamed the Serbs.
Gorazde, 30 miles southeast of Sarajevo, is surrounded by Serbian forces, the last government stronghold in eastern Bosnia. The population of 40,000 has been swollen by an estimated 30,000 ethnic Muslim refugees.
In London, the Foreign Office said Britain was planning to bring together "the principal governments and parties concerned, the United Nations and other organizations to try and promote a peaceful settlement in Yugoslavia."
Britain holds the rotating presidency of the European Community, which has sought in vain to end the fighting between Serb, Croat and Muslim communities in Bosnia.
The new diplomatic initiative runs parallel with peace talks called by Lord Carrington, the EC's chief negotiator on Yugoslavia.