Floods that caused billions of dollars (euros) in losses were the world's most expensive natural disasters so far this year, with central Europe being hit hardest, reinsurers Munich Re said on Tuesday.

Altogether, natural catastrophes -- also including earthquakes, tornados and heat waves -- caused $45 billion (35 billion euros) in losses in the first half of 2013, well below the 10-year average of $85 billion.

Insured losses worldwide totalled about $13 billion, said Munich Re.

Inland flooding that affected parts of Europe, Asia, Canada and Australia caused about 47 percent of overall global losses and 45 percent of insured losses, said the leading reinsurance company based in Munich, Germany.

The deadliest disaster out of 460 recorded "natural hazard events" worldwide was a series of flash floods in northern India and Nepal that killed more than 1,000 people in June after early and exceptionally heavy monsoon rains.

By far the most expensive natural disaster was the river flooding that hit southern and eastern Germany and neighbouring countries in May and June, causing more than $16 billion in damage, most of it in Germany.

"The frequency of flood events in Germany and central Europe has increased by a factor of two since 1980," said Munich Re board member Torsten Jeworrek.

In some places, 400 litres of rain per square metre fell within a few days. With the ground already saturated from the rainiest spring in half a century, this led to rapid swelling of the Danube and Elbe river systems.

Peter Hoeppe, head of Munich Re's Geo Risks Research unit, said in a statement that "it is evident that days with weather conditions that lead to such flooding are becoming more frequent".

As weather systems tend to remain stationary for longer, he said there was a higher chance for heavy and long-lasting rains, and for summer heatwaves and droughts.

"Debate in climate research is currently focusing on what the causes of such changes in weather patterns could be and what role climate change might play in this," he said. "But it is naturally not possible to explain single events on this basis."

The second most expensive disaster was a series of severe tornadoes that hit the United States in May, especially a maximum-strength tornado that devastated the Oklahoma City suburb of Moore.

The tornado, which killed 26 people, left more than 270 injured and in Moore alone completely destroyed more than 1,000 buildings, caused an overall economic loss of more than three billion dollars, about half of it insured.

"The central states of the USA have the highest tornado risk in the world," said Hoeppe. "Altogether, however, the US tornado season has been below average so far: by the end of June, 625 tornadoes had occurred, compared with the longer-term average of 1,075."