Efforts to reach a political agreement on Iran's nuclear program by the end of the month intensified on Tuesday as negotiations between the United States and Iran resumed, and Western officials warned that tough issues remained unresolved.
Senior European officials were expected in Lausanne later on Tuesday, with foreign ministers possibly joining them at the end of the week if talks advance.
A Western diplomat said "we're still not there yet" on reaching an end-March deadline for a framework agreement between Iran and six major powers -- the United States, Britain, France, Germany, Russia and China.
The parties have set a June 30 deadline to finalize all the technical details of an accord. Western officials say privately that overcoming disagreements on some of the remaining sticking points would be very difficult.
Sticking points include the level of Iran's enrichment activities and how sanctions would be lifted.
Iran's Oil Minister Bijan Zanganeh said "unjust" Western sanctions should be lifted, the official IRNA news agency reported on Monday.
He said Iran was ready to increase oil exports by up to 1 million bpd when sanctions are lifted and this would not have an impact on crude oil prices.