"I don't think the construction of protective reactor shells can be avoided but the question is what they will be like," Leonid Bolshakov, director of the Nuclear Energy Safety Institute, said in an interview with RIA Novosti.

Yet it is too soon to put the Fukushima accident in the same league with Chernobyl, he added.

"A total of 100 kilocuries of radioactive material has been released in Japan. I'd like to remind you that in the Chernobyl accident 50 megacuries were released," he said.

Other experts agree that the sarcophagus might be the best option to contain the accident.

"That's probably right," said Hugh Price, a retired Compliance Officer with EDF Energy.

"The fuel has partly melted, and escaped through the breach in the donut under the reactor, and the containment area. That has led to the high level of contamination outside the reactor," he said.

"What to do about it depends on the costs among other factors. It might be cheaper to flood the place with boronated water to soak up all the loose neutrons, as in a refueling operation, and then use remote vehicles to pick up all the fuel, but that won't be easy if it has all melted. So a sarcophagus is a likely option but probably not the only one."

The Fukushima Daiichi operator said earlier on Tuesday the consequences of the Fukushima accident could ultimately exceed the level of Chernobyl.

Officials from the Nuclear and Industrial Safety Agency (NISA) announced that the crisis level had been raised on the international nuclear and radiological event (INES) scale, putting the emergency on a par with Chernobyl.

"The radiation leak has not stopped completely and our concern is that the amount of leakage [of radiation] could eventually reach that of Chernobyl or exceed it," Kyodo news agency quoted the official from TEPCO as saying.

NISA said that Japan raised the severity level of the accident on Tuesday from the current 5 at the plant to the maximum 7 on an international scale, which the Chernobyl disaster was given in the former Soviet republic of Ukraine in 1986. The Chernobyl plant was previously the only nuclear disaster that had ever been rated at level 7.

A powerful quake and tsunami hit northeastern Japan on March 11 leaving more than 27,000 people dead or missing and disabling the cooling systems at Fukushima reactors. Radioactive elements were later found in the water, air and food products in some parts of Japan.