The United States and Europe have begun preparing to lift the trade sanctions that have hobbled the Iranian economy, as a historic nuclear deal came into effect.

The procedure to lift the embargo began 90 days after the UN Security Council endorsed the accord signed in Vienna in July, a milestone referred to as "Adoption Day." 

But foreign firms will not be able to resume ties with Iran's oil industry and banks right away -- sanctions will remain in place until Iran fulfills its end of the bargain. 

The next stage in the process -- "implementation day" -- will only come when UN nuclear watchdog the IAEA confirms Iran has dramatically scaled back its nuclear programme. 

Iran said that lengthy process will probably start this week.