Officials from the US, UK, France, China, Russia and Germany hope the
talks, in the Turkish city of Istanbul, halt rising tensions in the
region.
Iran says its nuclear programme is peaceful, but critics suspect it of seeking to develop nuclear weapons.
Israel has hinted in recent months that it could carry out a pre-emptive strike on Iran to prevent that happening.
The talks are the first to be held between Iran and the world's major powers for 15 months.
On Thursday, Iranian President Mahmoud Ahmadinejad said his
country was "standing firm on its fundamental rights and under the
harshest pressure will not retreat an iota from its undeniable right".
'Renewing confidence'
Few major developments are expected in Istanbul, but the six
nations, known collectively as the P5+1, hope it will reopen
negotiations and cool tensions.
They have
urged Tehran to engage seriously in the talks, with US President Barack
Obama describing it as the "last chance" for diplomacy to work.
Russia said the talks had to be "constructive", and warned both sides not to "overblow the differences" between them.
"We really need to find a middle course," said Deputy Foreign
Minister Sergei Ryabkov. "The negotiations are about renewing
confidence."
US Secretary of State Hillary Clinton said Iran had to
"demonstrate clearly in the actions they propose that they have truly
abandoned any nuclear weapons ambition".
The P5+1 hope eventually
to persuade Iran to reduce its enrichment of uranium and fully open up
its nuclear facilities to inspectors from the International Atomic
Energy Agency (IAEA).
There are suggestions that the stringent sanctions on Iran could be reduced if it complies with the requests.
The last series of international talks broke down in January 2011 after the parties failed to agree on any issues.
Since then, the IAEA expressed concern that Iran had failed
to co-operate with its inspectors and had carried out activities
"relevant to the development of a nuclear explosive device".
Israel, which believes a nuclear-capable Iran would be a
direct threat to its security, has warned that time is running out to
prevent that outcome.
Prime Minister Benjamin Netanyahu has said he would never
allow Israelis to "live in the shadow of annihilation", and hinted his
country is ready to strike Iran's nuclear facilities if diplomacy does
not work soon.
US President Barack Obama has warned against "loose talk of war", while stressing that all options remain open.
Source: BBC News