WASHINGTON — The United States Wednesday rebuffed an Iranian warning that new nuclear talks would fail if they were used to exert pressure, demanding assurances Tehran was not building an atomic bomb.

"We will demand that Iran live up to its international obligations -- that it provide verifiable assurances it is not pursuing a nuclear weapon," White House spokesman Jay Carney told reporters aboard Air Force One.

The warning came after Iranian parliamentary speaker Ali Larijani warned that the talks offered by the United States, Russia, China, France, Britain and Germany would fail if they were used to "pressure" Tehran.

Carney said the United States was "clear-eyed" about its approach, given that Tehran declined to discuss its nuclear program in previous rounds of talks.

"We will not relent in our efforts through sanctions and other measures to isolate and pressure Iran," he said.

"Actions are what matter here, and we will judge Iran by its actions."

On Tuesday, European Union foreign policy chief Catherine Ashton, who represents the world powers, said she hoped for real progress in the talks at a time and place yet to be announced.

President Barack Obama meanwhile said he expected it would "quickly" become clear if Iran was serious about easing concerns about its nuclear intentions in the talks.

In a February 14 letter to Ashton, Iran's chief nuclear negotiator Saeed Jalili said Tehran was ready to resume the deadlocked negotiations at the "earliest" opportunity as long as the world powers respected its right to peaceful atomic energy.

At the last talks between the two sides held in Istanbul in January 2011, Iran refused to address questions on its nuclear program, laying down what diplomatic sources said were "pre-conditions" such as a lifting of sanctions.

The prospects of new talks come at a time of heightened tension between Iran and its regional arch-rival Israel, and as Tehran struggles under a punishing new range of US and European Union sanctions.

Western powers and Israel suspect Iran is seeking to build a nuclear bomb under the guise of a civilian atomic program, a charge consistently denied by Tehran, which says its nuclear drive is aimed for peaceful purposes.