· Altitude bomb to knock out electronic systems

France has secretly modified its nuclear arsenal to increase the strike range and accuracy of its weapons. The move comes weeks after President Jacques Chirac warned that states which threatened the country could face the "ultimate warning" of a nuclear retaliation.

A military source quoted yesterday by the Libération newspaper claimed France had tinkered with its nuclear weapons to improve their strike capability and make this threat more credible.

The source said there had been two major changes: the bombs can now be fired at high altitude to create an "electromagnetic impulsion" to destroy the enemy's computer and communications systems; and the number of nuclear warheads has been reduced to increase the missiles' range and precision.

During his surprise speech, which was made in January, President Chirac said: "The number of nuclear warheads has been reduced in certain of the missiles in our submarines".

Military experts said this was not a step towards disarmament, but a move to improve the performance of the weapons. Until now each submarine carried 16 French-made M45 missiles, each fitted with six nuclear warheads. After being fired, each warhead would separate to hit a different target, in effect giving each submarine 96 nuclear bombs.

In reducing the number of warheads, down to one per missile in some cases, the weapon is lighter and has a longer range. It can also be targeted more accurately.

Libération speculates that while potential targets are "secret", it is clear they include the Middle East or Asia, and that its military contacts suggest the changes are aimed at adding "flexibility" to France's nuclear deterrent.

"These evolutions are aimed at better taking into account the psychology of the enemy," defence minister Michèle Alliot-Marie said after President Chirac's warning in January.

In a speech to MPs, she added: "A potential enemy may think that France, given its principles, might hesitate to use the entire force of its nuclear arsenal against civilian populations.

"Our country has modified its capacity for action and from now on has the possibility to target the control centres of an eventual enemy."

French government sources said the president's speech, given at a nuclear submarine base in Brittany, was not targeted specifically at Iran - despite Tehran's decision to continue its nuclear programme - or at individual terrorist organisations, but at countries that posed a direct threat to France itself.

It is also seen as an attempt to justify the more than €3.5bn (£2.4bn) a year France spends to maintain its estimated 300-350 nuclear weapons more than a decade after the end of the cold war.

"The ultimate warning restores the principle of dissuasion," the military source told Libération. The president is not talking about a choice between an apocalypse or nothing at all."

The paper says according to its information "ultimate warning" could take two new forms.

The most demonstrative would be to fire a relatively weak warhead into a deserted zone far from centres of power and habitation. The more radical option would be to explode a bomb at an extremely high altitude with the aim of creating a brief but enormously strong electromagnetic field which would disable or destroy all non-protected electronic systems in the area.

During the cold war France's "ultimate threat" involved firing nuclear bombs into Soviet military divisions and large cities.