Iranian President Hassan Rouhani has vowed that his government would never develop nuclear weapons.

It is his strongest signal yet that he may be seeking a diplomatic thaw with the west after decades of acrimony.

In an interview with NBC News, the new Iranian president also insisted he had "complete authority" to negotiate a nuclear deal with the US and other western powers.

"We have time and again said that under no circumstances would we seek any weapons of mass destruction, including nuclear weapons, nor will we ever," Mr Rouhani said when asked whether he would forswear nuclear arms.

Mr Rouhani's conciliatory comments appeared to be another sign of his willingness to work towards a diplomatic solution in Iran's bitter nuclear standoff with the west.

The US and its allies are intrigued but still wary, making clear they hope to see tangible steps to back up his words.

Speaking to the US network at his presidential compound in Tehran, Mr Rouhani said the tone of a letter he had received from US President Barack Obama, part of a recent exchange of messages between the leaders, was "positive and constructive".

"It could be subtle and tiny steps for a very important future," Mr Rouhani said six days before he is due to address the UN General Assembly.

Iran's supreme leader Ayatollah Ali Khamenei has said that nuclear weapons development would be inconsistent with Islamic values.

But a willingness by a newly-elected president to rule out nuclear arms could help provide a new opening in long-stalled international nuclear talks.