Iran's Ayatollah Ali Khamenei urged his armed forces Sunday to increase their "defensive preparedness", denouncing a US warning that military action is an option if there is no nuclear deal.

In a speech to commanders and troops, the supreme leader said "the other side with insolence threaten us all the time", denying Iran was seeking an atomic bomb and insisting its military doctrine is defensive.

Khamenei's remarks came after General Martin Dempsey, chairman of the US Joint Chiefs of Staff, reiterated last week that should nuclear talks with Iran fail "the military option... is intact".

The United States has long said bombing Iran's nuclear sites and other key facilities may be necessary if Tehran does not rein in its atomic activities.

Khamenei did not name Dempsey, but said: "After a period of silence by the other side, one of its officials has once again recently talked of options on the table.

"On the one hand they bluff, and on the other hand they say the Islamic Republic of Iran should halt its defensive advancements, which is a stupid remark.

"Iran will never accept such stupid remarks and the nation has proved that if it is attacked, it will defend itself quite powerfully. It will stand united and like a strong fist against illogical aggressors."

Iran and six world powers led by Washington agreed on April 2 on the key parameters of a nuclear agreement that could end the 12-year standoff over Tehran's atomic programme.

A comprehensive agreement is due by June 30 but Khamenei has stressed that concrete details, including how to lift sanctions, have not yet been decided.

"All bodies from the ministry of defence to the army and the Sepah (Revolutionary Guards) should increase their military and defensive preparedness. This should be regarded as an official directive," he said.

Russia, one power in the nuclear talks, last week lifted a ban on the sale of its S-300 missile systems to Iran, a move condemned by Israel which vehemently and frequently warns of a bad nuclear deal.

- Iran 'no threat' to region -

Russia said progress in the talks meant there was no longer a need for it to prohibit export of the S-300 surface-to-air missiles.

Iran, which hailed Russia's move as a step toward "lasting security" in the region, currently lacks advanced air defences that could knock out modern fighter aircraft in the US or Israeli air forces.

Opponents of a nuclear deal, including Gulf states, fear that Iran -- fuelled by money released if sanctions are lifted -- could be emboldened into furthering its regional aims.

Iran, the main Shiite power, is a key player in several conflicts, backing President Bashar al-Assad against rebels in Syria and supporting Iraq's government against the Islamic State group.

It has also been accused of backing Shiite Huthi rebels who have seized control of large parts of Yemen, prompting a Saudi-led air war in support of the country's embattled government.

But Khamenei said Iran was not meddling nor a danger to the Middle East, instead taking aim at the United States, other Western powers and Israel for creating conflict.

"The Islamic Republic of Iran, despite upgrading its defensive and military capabilities, shall never be a threat to regional and neighbouring countries," he said.

"Americans as well as Europeans and some other sycophants have made up the myth of the nuclear issue and nuclear weapons to say the Islamic Republic is a threat.

"Today the biggest threats for the region and the world are America and the Zionist regime who... interfere in any place they deem it necessary and launch massacres."

Iran's Foreign Minister Mohammad Javad Zarif has also criticised US warnings by Dempsey and other officials such as Pentagon chief Ashton Carter, dismissing them Saturday as "an old habit that dies hard".

"The fact we are negotiating indicates that everybody understands the only way to deal with Iran is to recognise Iran's rights and have mutual respect," he said. "That will provide a far better answer than getting engaged in disastrous adventurism."