US Secretary of State John Kerry said on Sunday that there are significant differences between the West and Iran regarding a deal over Tehran's nuclear programme.
"Obviously we have some very significant gaps still, so we need to see if we can make some progress," Kerry told reporters in Vienna just before a meeting with European Union (EU) foreign policy chief Catherine Ashton, who is convening the talks.
Nuclear negotiations have been held in Vienna between the West and Iran for 10 days seeking to prevent Tehran from making atomic weapons.
"It is vital to make certain that Iran is not going to develop nuclear weapons, that their programme is peaceful. That's what we are here trying to achieve", he added.
Kerry arrived in Vienna earlier today, where he will hold non-official meetings with his British, French and German counterparts, for talks over the coming days to give a push to nuclear negotiations with Iran aiming at concluding a deal before a deadlines set for July 20.
"Positions are still far apart," and the ministers had come to "try and narrow differences," French Foreign Minister Laurent Fabius was quoted as saying.
China and Russia are sending lower-ranking officials.
The six world powers are seeking guarantees from Iran to confirm its nuclear programme is peaceful.
Doubts are rising over the possibility of concluding a deal by July 20, and an extension seems inevitable.
Iran insists its programme is only for peaceful energy production and medical research, while the world powers fear it's a covert to produce nuclear weapons.