BELGRADE, Yugoslavia (AP) _ At the Institute for Prematurely Born Infants, anxious nurses shuttled from bed to bed today, hoping backup generators would keep their tiny patients warm and supplied with enough oxygen to stay alive.

A power outage caused by NATO airstrikes in the Yugoslav capital is threatening the survival of about 70 babies in the institute, the only hospital for premature infants in Yugoslavia.

When the Belgrade hospital lost power Sunday night following a NATO attack on a nearby power plant, officials immediately turned on the backup generator system. That provides enough power to keep the incubators and oxygen machines going, but not those that monitor vital functions.

At one point, the institute wasn't able to keep the oxygen supply going for some infants, forcing nurses to ventilate the tiny children mechanically, officials told reporters escorted to the hospital Monday. One little boy suffered severe problems when his oxygen supply failed.

``We have a backup system, but not all the time. Generators can maintain power for only two hours, three at best,'' said Dr. Slobodanka Ilic, the institute's director.

The hospital now has 111 infants, 70 of whom are in incubators that operate on electricity.

``If we want those babies to survive, we need a constant power supply,'' said Leposava Milicevic, the Serbian minister for public health. ``We are trying to do our best, but we can't help those you have seen very long.''

Because the institute now must rely on generators for electricity, it also cannot maintain constant temperature and humidity, important for such frail children.

Using generators has created another problem for Ilic.

NATO has targeted oil refineries, saying it aims to choke off fuel supplies that enable the Yugoslav army to continue its crackdown in Kosovo.

``We also need fuel for generators,'' Ilic said. ``And our fuel system has been destroyed.''