A new round of talks on Iran's nuclear programme being held in the Russian capital, Moscow, has yielded few signs of a breakthrough, reports say.
Russia's negotiator has said differences between Iran and the powers at the talks have proved "hard to reconcile", Russian media say.
The talks between Iran and six nations follows other recent inconclusive rounds in Istanbul and Baghdad.
The West suspects Iran is seeking nuclear weapons, a claim it denies.
Earlier this month, the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), said talks held with Iran in Vienna had made "no progress".
The latest negotiations between between representatives of Tehran and the so-called P5+1 (US, UK, China, Russia, France and Germany) took place in Moscow's Golden Ring hotel.
Following the talks, Russian news agency Interfax quoted Deputy Foreign Minister Sergei Ryabkov as saying: "The main stumbling block is that the sides' positions are rather difficult and tough to reconcile."
Michael Mann, spokesman for the P5+1, told reporters there was an "intense and tough" exchange of views during the day.
"We agreed to reflect overnight on each other's positions," he said.
It was in contrast to the start of the talks, when the respective lead negotiators - Saeed Jalili for Iran and Baroness Catherine Ashton for the P5+1 - nodded "hello" to one another and took their seats on opposite sides of a table separated by a bloom of white flowers.
Mr Mann told reporters then that the atmosphere was good and businesslike.
He said that the world powers were hoping that Iran would seriously engage with their concrete proposals.
These include a demand that Iran suspend enrichment of uranium to 20%, export its stockpile of 20%-enriched uranium and close down an underground enrichment facility near the city of Qom.
In return, the world powers say that they are prepared to start by offering help with nuclear safety measures.
Iran is seeking more though: it wants the West to lift sanctions, including an EU oil embargo and US measures against Iran's Central Bank.
It also says its "non-negotiable" right to enrich uranium must be recognised.