VIENNA – The control systems of Iran’s Bushehr nuclear plant have been penetrated by a computer worm unleashed last year, according to a foreign intelligence report that warns of a possible Chernobyl-like disaster once the site becomes fully operational.

Russia’s envoy to NATO, Dmitry Rogozin, also has raised the specter of the 1986 reactor explosion in Ukraine, but suggested last week that the danger had passed.

The report, drawn up by a nation closely monitoring Iran’s nuclear program, said such conclusions were premature and based on the “casual assessment” of Russian and Iranian scientists at Bushehr.

With control systems disabled by the virus, the reactor would have the force of a “small nuclear bomb,” it said.

“The minimum possible damage would be a meltdown of the reactor,” it says. “However, external damage and massive environmental destruction could also occur … similar to the Chernobyl disaster.”

The virus, known as Stuxnet, has the ability to send centrifuges spinning out of control and temporarily crippled Iran’s uranium enrichment program. It is believed to have been the work of Israel or the United States, two nations convinced that Iran wants to turn nuclear fuel into weapons-grade uranium.

Iran has acknowledged that the malware – malicious software designed to infiltrate computer systems – hit the laptops of technicians working at Bushehr, but has denied that the plant was affected or that Stuxnet was responsible for delays in the startup of the Russian-built reactor.

The Islamic Republic is reluctant to acknowledge setbacks to its nuclear activities, which it says are aimed at generating energy but are under U.N. sanctions because of concerns they could be channeled toward making weapons. Only after outside revelations that its enrichment program was temporarily disrupted late last year by the mysterious virus did Iranian officials acknowledge the incident.

Rogozin, the Russian envoy, described how engineers at Bushehr “saw on their screens that the systems were functioning normally, when in fact they were running out of control,” conjuring up a frightening dimension to the potential fallout from the virus.

“The virus which is very toxic, very dangerous, could have had very serious implications,” Rogozin told reporters, adding it “could have led to a new Chernobyl.”