MOSCOW (AP) _ Soviet construction workers building a vault around the ruined nuclear reactor at Chernobyl are being held up by a shortage of concrete, the Communist Party newspaper Pravda reported Wednesday.

The article was a rare departure from usually upbeat reporting about the Chernobyl cleanup and indicated that the restarting of two other reactors at the complex could be delayed.

Pravda did not say whether radiation was still leaking from the wrecked No. 4 reactor, which was ripped open in an explosion on April 26.

The article said two new plants producing concrete for the reactor tomb are not working at full capacity, but it gave no reason for the problem.

Concrete is formed by mixing cement, sand, and gravel, to which water is added at the construction site. It was not immediately clear which of those elements, if any, was lacking at the concrete plants.

''The construction of this sarcophagus is going on slower than one would like,'' the newspaper said.

It said reactivation of reactors 1 and 2 and repair of a third unit next to the No. 4 reactor depended on completion of the vault.

The concrete shell also is to include cooling and monitoring equipment.

Soviet officials have said they plan to restart reactors 1 and 2 in October. The future of reactor No. 3 remains uncertain.

An English-language summary of the Pravda report issued by the official news agency Tass omitted mention of the delay, saying encasement of the No. 4 reactor was ''proceeding as planned.''

Pravda complained that the concrete shortage was also slowing construction of new housing for Chernobyl workers 25 miles south of the reactor complex. Chernobyl is 80 miles north of Kiev.

The workers will be needed to run reactors 1 and 2 when they are restarted.

The official death toll from the disaster is 28, but the recent appearance of two new graves of Chernobyl victims at a suburban Moscow cemetery suggested it has climbed to 30.

The government estimates the disaster caused the equivalent of $2.8 billion in direct damage. More than 100,000 people were evacuated from an 18-mile zone around the plant, and it appears many of them will never return home.

Soviet officials have said the disaster resulted from poorly planned and unauthorized experiments on an electricity-generating turbine.

Several implicated officials have been fired, and the former chief engineer of the Chernobyl plant was expelled from the Communist Party.

The ruling Politburo said in a July 19 report that those responsible for the accident will be prosecuted.