US engagement with Iran must convince Tehran to halt progress in its ability to build a nuclear bomb and accept tough international controls, according to a US Senate report out Thursday.

"The ultimate solution to the conundrum of Iran?s nuclear ambitions is not technical, but political," investigators told the Senate Foreign Relations Committee, which is led by Democratic Senator John Kerry.

"Only a political decision by the country?s leaders is likely to prevent Iran from someday producing a nuclear weapon. And that decision is inherently reversible," as long as Iran retains the ability to enrich uranium, they said in the 11-page body of the report.

The document came after Kerry held a series of hearings on Obama''s planned engagement with the Islamic republic, a move the Massachusetts senator and former presidential hopeful supports.

"At a minimum, one goal of the administration''s strategy on Iran should be to provide the right balance of pressure and opportunity to persuade the regime to agree not to take any further steps toward enhancing its capability to build a bomb and to accept strict verification standards," the report said.

Kerry said the document aimed to address some of the suspicions that Tehran seeks a nuclear arsenal under cover of what it insists is a civilian atomic energy program, which he called a key obstacle to better US-Iran ties.