'N Korean nuke much stronger this time'

The nuclear device detonated by North Korea this week was about five times more powerful than the country's maiden 2006 test, US seismologists said.

Scientists at Columbia University's Lamont-Doherty Earth Observatory said they based their conclusions on data collected from seismic stations, including one in China.

"The second test appears to have a significantly higher yield" than the one in 2006, said Paul Richards, an expert on seismic detection of nuclear tests.

Another Columbia expert, Won-Young Kim who co-authored with Richards a report on the 2006 blast estimated the power of the Monday test at between 2.2 and four kilotons, the university said in a statement.

The blast produced shaking equivalent to an earthquake of magnitude 4.5 to 4.7, the statement read.

According to the Columbia experts, North Korea's 2006 test bomb was estimated to be less than one kiloton in strength. Each kiloton is the equivalent of 1,000 tons of TNT.

For comparison, the bombs that nuclear powers currently have in their arsenal are of around 50 megatons in strength, while the bomb the US military dropped on Hiroshima in 1945 was of 13 kilotons.