Iran's president said the Islamic Republic has decided not to develop nuclear weapons out of principle, not only because it is asked to do so by treaties.

Hassan Rouhani told defence ministry officials on Saturday that if Iran wanted weapons of mass destruction, it would be easier for it to make chemical or biological weapons.

Even if there were no NPT or other treaties, our belief, our faith, our religion and principles tell us not to seek weapons of mass destruction. Hassan Rouhani, President of Islamic Republic of Iran

Rouhani was reiterating a policy set by Iran's Supreme Leader Ayatollah Ali Khamenei, who issued a religious decree banning the production and use of nuclear weapons. He has said holding such arms is a sin as well as "useless, harmful and dangerous".

"We are not after weapons of mass destruction. That's our red line," Rouhani said. "If Iran was after weapons of mass destruction, it would build chemical weapons. Those are easier to make. It would build biological arms, which are even easier than making chemical weapons."

He said Iran's "beliefs" and commitment to "ethical principles", not merely the UN's nuclear non-proliferation treaty, prevent it from making a bomb. Iran is a signatory to the NPT and says it will remain committed to its obligations not to build nuclear weapons under the treaty, but will not compromise on its right to enrich uranium and produce nuclear fuel.

"We signed these treaties to show the world we are not after such weapons," he told military commanders. "Even if there were no NPT or other treaties, our belief, our faith, our religion and principles tell us not to seek weapons of mass destruction."

Policy of moderation

The US and its allies have expressed fear that Iran seeks to develop a nuclear weapon. Iran denies the charge, saying its nuclear programme is legal, peaceful, and geared towards generating electricity and producing radioisotopes to treat cancer patients.

Rouhani said his government's policy of moderation and easing tensions with the outside world is "not a tactic" but a genuine change in the Islamic Republic's foreign policy.

"The foreign policy of the Islamic Republic of Iran is based on easing tensions and building confidence with the world. This is not a tactic or slogan. Iran is not seeking tensions with others...but we don't compromise on our dignity, independence, national interests and values," he said.

This policy, also supported by Khamenei, led to an historic interim nuclear deal with world powers on November 24 in Geneva. Iran stopped enriching uranium to 20 percent and started neutralising its existing stockpile of that grade in January in order to fulfil commitments reached under the deal. The US and the European Union also lifted some sanctions.

Iran and the six-nation group, including the five permanent members of the UN Security Council plus Germany, began talks earlier this month for a comprehensive deal in Vienna.