A recent report by London's International Institute for Strategic Studies (IISS) says Iran is developing a dozen ballistic missiles that can fly up to 2,000 kilometres (1,200 miles) and deliver a 450-1,200 kilo (990-2,640 pounds) payload.

Since 2006, the UN Security Council has adopted several coercive resolutions aimed at preventing any bid by Iran to develop a missile programme that could deliver nuclear weapons, although this goal has always been denied by Tehran.

The sanctions were suspended in July 2015 when the Security Council unanimously passed Resolution 2231 to endorse the hard-won nuclear deal struck days earlier between Iran and six world powers.

President Donald Trump is due to announce Tuesday whether the United States will remain in the deal that lifted international sanctions in exchange for curbs to Iran's nuclear programme.

Trump insists the deal, also signed by France, Britain, China, Germany and Russia, was "very badly negotiated" by the previous administration of Barack Obama and fails to address Iran's ballistic missile programme and regional interventions.

European leaders have pleaded with Trump not to withdraw from the deal, with France proposing new negotiations with Iran, although Tehran has refused to make any modifications to the agreement.

- Missiles and defence -

Iran considers missiles central to its defence against regional foes, namely Saudi Arabia and Israel, said IISS researcher Clement Therme.

It says they were never part of the nuclear deal and considers the issue to be non-negotiable.

Its critics say the missiles are offensive and could one day carry nuclear warheads.

They also accuse Tehran of arming Yemeni rebels who have fired missiles into Saudi territory.

Israel, which is in range of Iranian missiles, said the ballistic programme threatens its very existence.

- Historical factors -

Iran's determination to build a missile programme can be traced back to the brutal and destructive 1980-1988 war with neighbouring Iraq, said Therme.

"Tehran's ballistic missile programme forms part of its deterrence policy, particularly against possible military strikes on its nuclear installations," as has been threatened in the past by Israeli and US officials, said Therme.

With that in mind, Iran obtained an advanced Russian S-300 air defence system after the 2015 deal.

Iran's determination to develop ballistic technology is also linked to its ambition to master nuclear and military technologies and therefore be self-sufficient, or "khod-kafai" in Farsi, and less dependent on the outside world, Therme added.

- Are negotiations possible? -

Iran sees its military capabilities as central to its regional influence and revolutionary ideology, said Therme.

The Islamic republic is the key regional backer of the regime of Syrian President Bashar al-Assad in his country's seven-year war against rebels who have been trying to unseat him.

It is also a staunch opponent of Israel.

"Although moderate President Hassan Rouhani could be in favour of shedding revolutionary ideology on these (foreign policy) issues, he is not effectively in charge," said Therme.

In Iran power over international issues rests ultimately with the powerful supreme leader Ayatollah Ali Khamenei, and increasingly with the country's elite Revolutionary Guards who are conducting interventions abroad.

However, Khamenei did approve the nuclear deal in 2015 and said he was willing to see it as the basis for further negotiations if the US reciprocated, showing that he is willing to take pragmatic diplomatic steps.

"If Iran were to adopt a regional policy that was less hostile to Western interests, it would be possible to reach a compromise on its ballistic missile programme," said Therme.