Iran has further boosted its capacity for uranium enrichment, according to a report from the UN's nuclear agency.
The International Atomic Energy Agency (IAEA) says Tehran has now installed more than 1,000 advanced centrifuges at its Natanz enrichment plant.
It comes as the IAEA says it will resume talks with Iran on 27 September - the first such negotiations since President Hassan Rouhani was elected.
The West fears Iran wants to develop nuclear weapons - a claim it denies.
The IAEA's quarterly report says there has not been much growth in the production of the most sensitive nuclear material - uranium enriched to 20%.
However it says Iran has installed 1,008 advanced IR2m centrifuges at its Natanz plant compared with almost 700 in May.
It also says the Islamic state has begun making nuclear fuel for its planned Arak heavy-water research reactor, although it has postponed the commissioning of the reactor beyond the planned first quarter of 2014.
The report again raised concerns about the Parchin site, a suspected nuclear weapons development facility that investigators have been unable to fully inspect.
It also says the IAEA has still not got answers on outstanding issues about Iran's nuclear programme, "including those related to possible military dimensions".
Iranian President Hassan Rouhani has pledged to pursue "serious" talks with the West to ease tensions - and economic sanctions - over its controversial nuclear programme.
Lack of progress
Commentators point out that much of the latest technological developments will have taken place before Mr Rouhani took office on 3 August.
The country has been the target of four rounds of UN sanctions and numerous UN Security Council resolutions calling on it to cease enrichment work amid fears it aims to develop nuclear weapons.
Iran has refused to obey, saying it intends to enrich only for power station fuel or other peaceful purposes.
It already has about 18,000 centrifuges for uranium enrichment in operation but they are of the older IR1 type. The new IR2m machines are said to be faster and more effective.
Uranium enriched to 90% is required for a nuclear weapon.
Iran's talks with the IAEA were apparently held up by the country's presidential elections in June.
No negotiations between the two have taken place since the last quarterly report in May.
In June, IAEA director general Yukiya Amano complained of a lack of progress during 10 rounds of negotiations between Iran and the so-called P5+1 - the US, UK, France, Russia, China plus Germany - despite intensified discussions.