Former US officials called on the White House Wednesday to initiate direct talks with Iran over its nuclear program instead of relying solely on sanctions to persuade Tehran to change course.

The 35 prominent ex-diplomats, military officers and other officials from both political parties issued a report urging President Barack Obama to renew diplomatic efforts to defuse the crisis and take advantage of economic sanctions that have hit Iran hard.

"The United States should now dedicate as much energy and creativity to negotiating directly with Iran as it has to assembling a broad international coalition to pressure and isolate Iran," said the report from the Iran Project, a non-partisan group.

"Only by taking such a rebalanced approach might the United States achieve its objectives with respect to Iran's nuclear program," it said.

Those who signed the report included former Republican senator Richard Lugar; Michael Hayden, ex-CIA director under president George W. Bush; Ryan Crocker, the former US ambassador to Iraq and Afghanistan, as well as former national security adviser Zbigniew Brzezinski.

Direct talks with Iran would not replace current efforts by major powers to engage Tehran but would complement diplomacy by the five permanent UN Security Council members plus Germany, known collectively as the P5+1, the report said.

The last round of P5+1 talks with Iran in Almaty on April 5-6 failed to produce major progress.

Tightened sanctions have successfully piled pressure on Iran and may have slowed progress in its nuclear program. But the pressure has failed to stop Tehran's controversial uranium enrichment work, the authors said.

The US-led pressure, however, may harden Iran's resistance, contribute to an increase in repression and sow the seeds of "long-term alienation" between Americans and Iranians due to hardships produced by the sanctions, the report said.

"After 30 years of sanctioning and trying to isolate Iran, it seems doubtful that pressure alone will change the decisions of Iran's leaders," it said.

Direct negotiations could offer some relief from sanctions as part of a deal, in return for "verifiable Iranian commitments to greater transparency and agreed limits on Iran's nuclear program," the authors wrote.

Given the Iranian leadership's deeply held belief that Washington favors toppling the regime, any diplomatic progress would require the US government to take "active steps" -- beyond rhetorical gestures -- to convince Tehran it does not seek to overthrow the government, the report said.

The authors were apparently referring to espionage designed to undermine the Tehran regime.

As part of diplomacy on a nuclear deal, Washington and Tehran could also explore possible cooperation on Afghanistan and Iraq, counter drug trafficking efforts and arrange ways to prevent incidents from escalating into armed conflict, it said.