The United States has confirmed that it will take part in a meeting in Vienna next week on the Iran nuclear deal and offered to sit down directly with Tehran.

State Department spokesman Ned Price said Washington does not anticipate an immediate breakthrough as there will be difficult discussions ahead, but this is a healthy step forward.

Earlier, Iranian Foreign Minister Mohammad Javad Zarif in a tweet said the aim of the meeting in Vienna would be to finalise lifting of sanction on Iran.