MOJKOVAC, Yugoslavia (AP) _ A fragile dam wrapped in thin, white plastic is all that separates a poisonous mixture of lead, zinc and insecticides from the pristine Tara River in northern Montenegro.

A catastrophic collapse of the dam has been averted for now, but the future of the flood-damaged structure depends on foreign help and a continuation of unseasonably good weather.

The Tara dam, 38 miles north of the Montenegrin capital of Podgorica, is holding back a reservoir of toxic waste from nearby lead and zinc mines.

A break would send thousands of tons of the poisonous sludge into the Tara, probably killing all life in the river and other waterways it flows into, and threatening water supplies for millions of people.

The Tara feeds the Drina, which flows past Bosnia-Herzegovina to the Sava, a major tributary of the Danube. The endangered area stretches past Belgrade to the Black Sea, said Hanns Zimmermann of the U.N. Disaster Relief office in Geneva.

A United Nations team discovered the danger last week when it visited the dam after Montenegrin officials requested an exemption from U.N. sanctions so they could import materials urgently needed for repair work.

The United Nations imposed sweeping economic sanctions against Yugoslavia, now comprising only Serbia and Montenegro, for fomenting civil war in Bosnia- Herzegovina.

The U.N. Sanctions Committee on Wednesday gave emergency workers approval to ship repair supplies to Montenegro.

Montenegrin officials say the Tara river is the cleanest in Europe, and the U.N. experts visiting the site found that none of the poison had leaked through dam into the river.

''We were lucky that the dam resisted ... we virtually saved the river with plastic bags. It was really a millimeter from disaster,'' said Zoran Cetkovic, a young man who helped workers wrap the base in plastic after floodwater washed away concrete slabs protecting the dam's base.

The U.N. team plans to build three jetties and a small dam to keep the river from further eroding the base of the dam, which is holding back 122 million cubic feet of toxic waste.

Zimmermann organized the shipment of 1,500 wire cages from Italy so that engineers can make temporary protective structures at the dam, damaged by flash floods in October. The will be filled with rock and gravel.

The European Community has provided about $150,000 in emergency funds for the supplies.

''All that will surely help preserve the dam from further rainfalls, but a permanent solution must include the regulation of the Tara river course,'' dam inspector Vlado Kapor said Thursday.

Building a new structure to replace the 20-year-old dam would cost about $625,000, according to U.N. officials in Geneva, but that would still not solve the toxic waste problem.