Senior U.S. and Iranian officials said much hard work still needs to be done to bridge significant differences on an agreement to curb Iran's nuclear programme.
With a deadline approaching on Tuesday, each side sought to place the onus on the other to finalise a deal under which Iran would cut back its nuclear programme in exchange for relief from economic sanctions.
"If the other side...takes positive steps and does not make excessive demands, we will certainly reach a deal that benefits everyone," Iranian Foreign Minister Mohammad Javad Zarif was quoted as saying by state news agency IRNA shortly before he met U.S. Secretary of State John Kerry.
Significant differences remain, notably over the pace and timing of sanctions relief for Iran and the nature of monitoring mechanisms to ensure Tehran does not cheat on any agreement.
"The next few days will be extremely difficult," a senior Western diplomat said on condition of anonymity.
The talks may slide at least two or three days past the deadline, the diplomat said.
In addition to Iran and the United States, the talks include Britain, China, France, Germany and Russia. Foreign ministers from all the nations are expected in Vienna in the coming days to "check in" on the progress of negotiations.
The West fears that Iran has been trying to build a nuclear weapon but Iran says its programme is for peaceful purposes only.
