The Serb-led Yugoslav army Saturday pressed the civil war in breakaway Croatia, which said the federal forces had used surface-to-surface missiles for the first time in six months of fighting.
Yugoslav forces slammed the strategic town of Karlovac with a daylong bombardment and moved close enough to this Croatian capital to force a total blackout.
Saturday's attacks came as Serbian leaders prepared for a Monday conference designed to reform the shattered country.
The blackout in Zagreb was the first in the city and came after six distant but powerful explosions reverberated through the area. The all-clear was broadcast an hour later.
The Croatian High Command said the alert was caused by Soviet-made R-65 surface-to-surface missiles fired by the army at Croatian troops southeast of Zagreb.
Surface-to-surface missiles have never before been used in the six months of war between Croatian forces and the republic's army-backed Serbian minority, which bitterly opposes its bid for independence.
The command said the missiles had hit in the Turopolje area about 12 miles south of Zagreb, where Croatia's first-line defenses are deployed. A spokesman gave no details of casualties or damage.
The Croatian capital has not come under attack since federal planes targeted the offices of Croatian President Franjo Tudjman in September.
There were reports of rocket fire in the village of Kravarsko, just 20 miles south of the capital and 15 miles north of Karlovac, a center of Baroque architecture where fighting raged all day, leaving three dead and 19 wounded.
Thousands of people have died since Croatia and neighboring Slovenia declared independence from Yugoslavia in June. Croatia has lost more than a third of its territory to the army and Serb irregulars in the war.
Karlovac sits near the western border of the state dreamed of by Serbs and is important because of its supply and communications links with the Adriatic coast.
"It is a real war where the fighting does not stop," said a Radio Belgrade reporter accompanying the army side near Karlovac.
He said Croatian forces tried a tank and infantry attack from Karlovac but were repulsed with the loss of four tanks.
On the Croatian side, a duty officer at the town's crisis center said by telephone that the bombardment continued without break since morning and all parts of the town were under fire.
"Two or three bombs hit the center every 15 or 30 minutes," he said.
Milan Babic, a Serbian leader, said Monday's conference in Belgrade would seek to forge a "new Yugoslavia."
"Whether it will be a closer community or an alliance of sovereign states with a single currency, a common market and a certain level of armed forces, will be decided by the states which see their future interests in preserving the continuity of Yugoslavia," he told a Belgrade newspaper.
Germany has already recognized the independence of Croatia and Slovenia and full European Community recognition could follow Jan. 15 if they meet its conditions.
Macedonia and Bosnia-Herzegovina have also applied for recognition.