The six nations - the US, UK, Russia, China, Germany and France - want Iran to curb its nuclear programme in return for the lifting of UN sanctions.
Iran says it is not seeking nuclear weapons, but wants atomic energy.
It says its nuclear programme is solely designed to provide peaceful, civilian projects.
On Sunday evening, the so-called P5+1 group and Iranian officials said they were discussing the possibility of extending the negotiations, as a comprehensive agreement was still far away.
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