ASSOCIATED PRESS WRITER

SAMIR KRILIC

SARAJEVO, Bosnia-Herzegovina (AP)—The ominous boom of heavy artillery returned to Sarajevo on Thursday, and an aid convoy was canceled in the north as Bosnia approached another winter of war.

State-run radio said about 150,000 Muslim civilians in Maglaj in the north, cut off since June by Serb and Croat forces, begged the Muslim-led government:

‘‘Maglaj is expecting urgent help from you, because soon you won’t have anybody to help here.’‘

U.N. relief officials said they had to scrub the relief convoy intended for Maglaj and nearby Tesanj after Serbs refused to guarantee safe passage. Bosnian radio said both towns came under heavy shelling Thursday. That report could not be confirmed.

In Sarajevo, officials warned residents to stay inside as the city came under some of the worst shelling in weeks.

Lt. Col. Bill Aikman, a spokeman for U.N. peacekeeping forces in Sarajevo, said snipers shot at four U.N. vehicles within 36 hours, wounding an Egyptian soldier in the shoulder.

Thursday’s shelling appeared to be in retaliation for a mortar attack by government forces on the Serb-held northern suburb of Vogosca. ‘‘The Serbs decided to fire back with everything they’ve got,’‘ Aikman said.

Aikman said he did not know whether the shelling foreshadowed a new surge of fighting in Sarajevo, which has been relatively quiet recently.

But he added: ‘‘We are at risk here. Tensions are rising.’‘

In Vitez, a town in central Bosnia contested by Croat and government forces, an undetermined number of Muslim troops were killed in a huge blast.

Croats apparently mined trenches, then set off the charges after Muslim soldiers moved in, said officials at a local British U.N. base.

Bosnia’s war started 18 months ago when Serbs rebelled after the republic’s Muslims and Croats voted for independence from Serb-dominated Yugoslavia. The fighting has left up to 200,000 people dead or missing, and 70 percent of Bosnia in Serb hands.

A member of Bosnia’s collective presidency, Tatjana Ljujic-Mijadovic, expressed fear that Bosnia would suffer another winter of misery. Some 300,000 refugees lack shelter, and care is inadequate for the tens of thousands of prisoners, former prisoners and invalids who need it.

Bosnian Serb leader Radovan Karadzic and Serbian President Slobodan Milosevic pledged Thursday to maintain a cease-fire in Bosnia and allow ‘‘free passage’‘ of aid convoys through Serb territory, the Tanjug news agency reported.

But Bosnian Serbs said earlier they could not guarantee the safety of U.N. aid trucks headed for the Maglaj-Tesanj area. A separate Red Cross convoy was damaged by a land mine Wednesday just beyond the last Serb checkpoint.

U.N. officials said they would try again next week to get aid there.

The joint message by Karadzic and Milosevic blamed Bosnian President Alija Izetbegovic and his ‘‘extremist’‘ Muslim followers for prolonging the war. They said he did not represent most Bosnian Muslims, and called for renewed peace talks.

Izetbegovic and the Muslim-dominated parliament have rejected a peace plan that would carve Bosnia into three ministates, leaving the Muslims only small chunks of landlocked territory.