The United States is lobbying against an amendment to an international nuclear safety pact proposed by Switzerland, which Berne argues could help prevent Fukushima-style disasters but which may also increase industry costs, diplomats said.

Atomic energy powers Russia and Canada have also signaled opposition to the measure, which would put pressure on countries to upgrade existing nuclear plants and reach the safety requirements of new-generation reactors.

Washington says it wants to improve safety, too, but sees no need to change the 77-nation Convention on Nuclear Safety (CNS). It says Switzerland's initiative, tentatively backed by other European countries, could be counter-productive. It would not go into effect for many years and might not be ratified by all CNS states, it says.

A senior State Department official said about the proposal: "You are trying to drop a Ferrari engine into a Volkswagen. If you want a new car, let’s go to the show room" and buy one.

The diplomatic dispute highlights persistent differences on how to best make sure there is no repetition of the reactor meltdowns in Japan in early 2011 - the worst such accident since the one at Chernobyl a quarter of a century earlier.

Countries agree on the need for enhanced global safety after Fukushima, but not on how much international action is required.

For those against the Swiss proposal, "it is a matter of national sovereignty," said one European diplomat in Vienna, where the issue was debated by national experts last week before a decision-making diplomatic conference in February. It needs a two-thirds majority to be adopted.

The head of the International Atomic Energy Agency (IAEA) last month said progress was being made in strengthening nuclear safety in the world. "I have seen concrete improvements in safety features at every nuclear power plant I have visited since the Fukushima Daiichi accident," Yukiya Amano said.

But Switzerland - which, like Germany, decided to move away from nuclear power after Japan's emergency - says more is needed and seeks what it calls a culture of continuous improvement.

CNS states should not only apply up-to-date safety standards for new reactors, but also carry out back-fitting measures on plants that are already operating, it argues.

The convention was negotiated after the 1986 Soviet reactor explosion at Chernobyl spewed radioactive dust across much of Europe. It aims to foster high safety by enabling countries to scrutinize and question how others meet their obligations.

The Swiss draft says nuclear plants "shall be designed and constructed with the objectives of preventing accidents ... In order to identify and implement appropriate safety improvements, these objectives shall also be applied at existing plants."

The senior U.S. State Department official warned that the amendment would "not work, it will be divisive, and it will fundamentally damage" the atomic safety convention. "We will be tied up with this controversy for the foreseeable future instead of working with real problems," the official added.