The United States on Thursday denied a report that Secretary of State Hillary Clinton told her Russian counterpart that upcoming talks on Iran's nuclear program were a "last chance" to resolve the long-running standoff.

"The secretary did not send a warning to the Iranians through Foreign Minister (Sergei) Lavrov," State Department spokeswoman Victoria Nuland told reporters when asked about the report in Russia's Kommersant daily.

"The conversation that they had on Iran centered around ensuring that these talks, as they start, are structured in a way... that they bring substantive results," Nuland said.

Clinton's meeting with Lavrov in New York Monday also focused on ensuring that the talks "can't be used for stalling and that they can't be used for covering continuing activity" in Iran's nuclear program, Nuland said.

In her discussions with Lavrov on the sidelines of the UN Security Council, Clinton never used the adjective "last" to describe the opportunity that the planned talks represent, Nuland said.

The United States, Russia, China, Britain, France and Germany have agreed to resume stalled talks with Iran over its nuclear program at a time and place yet to be determined.

Kommersant reported Wednesday that Clinton had told Lavrov the talks were a "last chance" to resolve the crisis, and asked her Russian colleague to make this clear to the Iranian authorities."

Washington, which has no diplomatic relations with the Islamic Republic, has stressed there is still time to resolve the standoff over Iran's disputed nuclear program, but has refused to rule out a military strike as an option.

Israel, increasingly impatient with Iran's refusal to come clean on its nuclear program, also has refused to rule out military action.

The United States and Israel fear that Iran's uranium enrichment program masks a drive to build a nuclear bomb. Iran denies the charges, insisting its program is entirely peaceful.