A fire broke out Sunday at a Moscow nuclear research center that houses a nonoperational 60-year-old atomic reactor, an emergency official said.

While Russia's nuclear agency said there were no open flames and no threat of a radiation leak, the environmental group Greenpeace Russia expressed serious concern about the incident.

The fire was in a basement at the Institute of Theoretical and Experimental Physics in southwestern Moscow, said Sergei Vlasov, spokesman for the Moscow branch of the Emergency Situations Ministry. He said no casualties were reported.

Gray smoke rose above the institute, which is encircled by a wall, and an acrid smell filled the air. Some 30 emergency vehicles, including fire trucks and ambulances, stood inside and outside the main gate, witnesses said.

Sergei Novikov, spokesman for nuclear agency Rosatom, said there were no open flames, only smoke that came from an area housing power cables and could not affect any nuclear materials at the institute.

"This case poses no threat to fissile materials," said Novikov, adding that firefighters were pumping foam into the affected area. He said the institute's heavy-water research reactor was no longer operational.

A Greenpeace Russia official said the incident was potentially very dangerous.

"This is extremely dangerous … this should not have happened at all, but since it did, it shows that there has been a major failure in their operations," said Ivan Blokov, campaign director at Greenpeace Russia.

"What we have here is a large amount of radioactive material right in the center of Moscow, and even if a minor quantity leaks it would pose a serious problem," he said.

Interfax cited a police source as saying fire brigades were denied access to the facility for "a long time" before being allowed in.

Vlasov said he could not confirm the report, but said the fire had not been extinguished as of 2:45 p.m. RIA-Novosti reported earlier that the fire had already been put out.

The institute houses the Soviet Union's first heavy-water reactor, designed in the late 1940s, according to its website. Several phone calls to the institute went unanswered.