But authorities will not know for another decade exactly where to invest the money, said Andrea Goetz, deputy director of the Swiss Federal Office for Water and Geology.

The problem, Goetz told the NZZ am Sonntag newspaper, is that experts are still trying complete a map of the country's threatened areas.

Construction is already restricted in danger areas which have been mapped so far, to reduce the risk of death, injury and destruction of buildings and infrastructure.

"It is certain that risk zones will show up, ones which so far have by chance remained unaffected," Goetz was quoted as saying in an interview.

Mapping efforts so far have identified water courses where the risk of floods can be lowered.

The huge investment is worth it, Goetz said.

He pointed to a 26 million Swiss franc (17 million euro, 21 million dollar) project along the Aa river at Engelberg.

The central Swiss resort was hit hard by last week's floods, which were fuelled by torrential rain. With roads washed away, helicopters were needed to fly in supplies or evacuate residents and tourists.

Insurers are still counting the cost, but Goetz said the flood protection work had probably cut 100 million Swiss francs from the final bill.

Updated anti-flood measures along the Rhone river, which flows from an Alpine glacier and across a swathe of southwest Switzerland before reaching France, are expected to cost a billion Swiss francs.

Aging protection systems along the Rhine and the Linth, in central Switzerland, also need updating.

Currently, the federal budget for anti-flood measures is 60 million Swiss francs a year. The country's 26 cantons, or states, also spend their own funds.

In a separate interview Sunday, transport and environment minister Moritz Leuenberger said it was time for a rethink.

"It's a question of allowing more space for water courses and of stopping using risk zones," he was quoted as saying by the Le Matin Dimanche newspaper.

"With global warming, such events are to be expected. So it's important to continue our policy of reducing C02 emissions and to achieve our targets."

As policymakers pondered what to do in the future, rescue workers continued clearing debris in Switzerland's flood damaged regions.

Religious services were held in several battered communities, including Brienz near the capital Bern, where two people were killed last week.

Some evacuated residents returned to nearby Oey-Diemtigen, but authorities said up to 70 people would have to wait weeks if not months before their homes would be fit to live in again.

In Bern, some 340 residents of the low-lying Matte district along the Aare river - which burst its banks last week - were able to go home. But many others remained in temporary accommodation because their blocks still lacked electricity and gas.

In the central Lucerne and Obwalden regions, residents were still forced to boil drinking water because of damage to supply pipes. Authorities urged people to use water sparingly.

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