US President Donald Trump on Friday declared a more aggressive strategy on a nuclear deal between Iran and six world powers but stopped short of withdrawing from the 2015 deal.

Struck in Vienna by the five permanent members of the UN Security Council -- Britain, China, France, Russia and the United States -- plus Germany, the deal established controls to prevent Tehran from developing an atom bomb.

 

It was a breakthrough that ended a 12-year standoff with the West over Iran's disputed nuclear programme, and led to a partial lifting of international sanctions on Tehran.

Here is some background about the deal:

- 21 months of talks -

Talks on Iran's nuclear programme start in 2013 after newly elected Iranian President Hassan Rouhani gives the go-head, with the agreement of the country's supreme leader Ayatollah Ali Khamenei.

By November, an interim deal is agreed, freezing some of Iran's nuclear activities in exchange for minor sanctions relief.

It is finalised in April 2015 and signed in Vienna on July 14 that year, ending 12 years of crisis and 21 months of negotiations.

The deal is adopted by the UN Security Council on July 20, 2015 and comes into force on January 16, 2016.

- Main points -

The accord brings to a minimum of one year, for at least 10 years, the "breakout time" that Iran needs to produce enough fissile material to make an atom bomb.

Tehran agrees to slash the number of uranium centrifuges, which can enrich uranium for nuclear fuel as well as for nuclear weapons, from more than 19,000 to 5,060, maintaining this level for 10 years.

All enrichment is to take place at the Natanz facility only and Iran's pre-deal stockpile of 12 tonnes of low-enriched uranium -- enough for several nuclear weapons if further enriched -- is to be reduced to 300 kilogrammes (660 pounds) for 15 years.

Only enrichment to low purities is allowed, also for 15 years.

Iran's Arak reactor is to be redesigned so that it does not produce weapons-grade plutonium, the alternative to highly enriched uranium for a nuclear weapon.

- Controls -

The International Atomic Energy Agency (IAEA) is charged with regular inspections of facilities such as uranium mines and centrifuge workshops for up to 25 years.

The agency says in September that Tehran is sticking to the terms of the deal. Its staff had conducted at least 400 inspections of sites in Iran and 25 snap inspections, it says.

- Sanctions eased -

The deal paves the way for a partial lifting of international sanctions on Iran, opening the door for foreign investors, with French energy giant Total and carmakers PSA and Renault quick to strike deals.

UN embargoes on conventional arms and on ballistic missiles have been maintained up to 2020 and 2023 respectively.

- 'The worst deal'? -

Trump has railed against the deal struck by his predecessor and vowed to tear it up, deriding it as one agreed to out of "weakness".

At a much-anticipated White House speech on Friday, the US president said he was refusing to certify the deal and warned Washington may yet walk away from "one of the worst" agreements in history, leaving its fate in the hands of Congress.

The nuclear deal, Trump said, had failed to address Iranian subversion in its region and its illegal missile program.