Neither side offered any word on the progress of discussions, The New York Times reported Wednesday.
Both sides said the Istanbul talks, which started Tuesday and ended Wednesday, were technical discussions among lower-ranking experts representing Iran and the P5-plus-1 group -- the United States, China, Britain, France and Russia, the five permanent members of the U.N. Security Council, and Germany.
The talks Wednesday were conducted as rhetoric ratcheted up tensions in the Middle East. Iran repeated its threat to close the Strait of Hormuz in response to recently stepped-up U.S. and European sanctions directed at the Iranian oil industry, saying the new sanctions won't effect its resolve to prevail in the dispute over whether its nuclear program is developing weapons, as the West claims, or for civilian purposes, as Tehran maintains.
On Wednesday, the air force commander of Iran's elite Islamic Revolutionary Guards Corps said he had contingency plans to strike 35 U.S. bases in the early going of any potential conflict.
Catherine Ashton, the European Union's top foreign policy official and the lead negotiator for the Western group, said in a statement, "The experts explored positions on a number of technical subjects."
The statement said her deputy would meet with the deputy of lead Iranian negotiator Saeed Jalili.
Russian Deputy Foreign Minister Sergei Ryabkov said negotiators made some progress in their two days of discussions, Iran's government-backed Fars news agency reported.
"There are grounds to speak of certain progress," Ryabkov said, adding, "I cannot say that we reached some sort of breakthrough or achieved decisive progress."
The U.N. Security Council has demanded Iran end its uranium enrichment program, which Iran says it won't do because it has the right to enrich uranium for peaceful purposes as a signatory of the Treaty on the Non-Proliferation of Nuclear Weapons.
Sanctions by the United States and Europe already have cut into Iran's export of oil and led to economic disruptions and inflation, the Times said. Iranian President Mahmoud Ahmadinejad said Tuesday Tehran would resist the sanctions and will use them as a way to help the country reduce its dependence on oil sales.