Weather experts said on Wednesday that the tornado that hit Oklahoma on May 21, Tuesday, was stronger than the atomic bomb that hit the Japanese city of Hiroshima during World War II.

The brute force came from a combination of wind, humidity and rainfall that the National Weather Service in the U.S. gave its wind speed, breadth and severity of damage rating of EF-5.

The twister, which has damaged an estimated $3 billion worth of properties and lands in Oklahoma, had estimated wind speeds between 320 and 337 km/h.

Meteorologists, however, differ on the strength of the tornado that at some points was two kilometers wide and had a path that stretched for 27 kilometres and 40 minutes. They reckoned it was eight to over 600 times the strength of the A bomb that decimated Hiroshima.

While it was quite long for a regular tornado, National Severe Storms Laboratory research meteorologist Harold Brooks pointed out that less than one per cent of all U.S. tornadoes are that violent, and it occurs only about every decade.

Adam Houston, meteorology professor at the University of Nebraska, Lincoln, said that based on the climate history of twisters in May, they often cluster in a spot 160 kilometres wide in central Oklahoma. That spot provides the right balance for and mixture of warm weather condition, moist air and strong wind to create tornadoes.

Insurer QBE, which makes a third of its profit in the U.S. and suffered $170 million loss in 2012 due to claims against crops caused super storm Sandy, however, has a low exposure in Oklahoma.

But the estimated $3 billion insurance losses from Tuesday's twister hit in Oklahoma is significantly smaller compared to the estimated $13 billion insured losses from the 2011 Christchurch earthquake in New Zealand.

Analysts explain the lower financial costs in a tornado compared to a tremor to floods because of the concentration of damage on a smaller area.

The Oklahoma Department of Emergency Management has estimated fatalities at 24 people, including nine children, down from the initial report of 51 which double counted the victims.

While no survivors had been found in the rubble of the heavily damaged Plaza Towers Elementary School in Moore on Tuesday afternoon, state official said search efforts will continue.

Meanwhile, the Internal Revenue Service announced tax relief to individuals and business affected by the tragedy, specifically those from the Cleveland, Lincoln, McClain, Oklahoma and Pottawatomie counties.

The special tax relief measures include extending tax filing and payment deadlines after May 18, 2013 to Sept 30, 2013, and waiving failure-to-deposit penalties for federal payroll and excise tax deposits normally due on or after May 18 through June 30. But the deposits should be made by June 3.