US Secretary of State John Kerry on Wednesday "words are no substitute for actions" on the Iran nuclear issue and said it was too early to talk about easing sanctions on the country.

Speaking ahead of marathon talks in Rome with Israeli Prime Minister Benjamin Netanyahu, Kerry said US sanctions on Iran could be revoked only once it is clear the country's nuclear programme is peaceful.

"We do welcome the change of rhetoric, the change of tone, the diplomatic opening that the Iranians have offered," Kerry said, adding however: "We are adamant that words are no substitute for actions".

For any discussion over sanctions, he said: "We will need to know that actions are being taken which make it crystal clear, undeniably clear, failsafe to the world that whatever programme is pursued is indeed a peaceful programme."

At the same time he hailed the recent signs of openness in Iran following the election of President Hassan Rouhani and said the country should now respect the same rules as other nuclear powers.

"No deal is better than a bad deal," he said, adding: "President (Barack) Obama has made it very clear he will pursue a diplomatic initiative, but with eyes wide open".

Netanyahu agreed with Kerry on Iran saying Iran's nuclear capability was "the foremost security problem that we face", adding: "I think a partial deal that leaves Iran with these capabilities is a bad deal."

Nuclear talks between Iran and western powers are gathering steam after Iranian negotiators last week presented a still-confidential new proposal to the West aimed at ending the stand-off.

Enmity and suspicion between Iran and the United States stretch back decades to the Islamic revolution and the 444-day siege of the US embassy in Tehran in 1980. And any deal could be a tough sell to hardliners in both nations.

The next talks in Geneva are set for November 7-8.

The New York Times reported last week the White House may be considering unblocking some $10 million in frozen Iranian assets in the United States to give the Rouhani leadership some access to badly needed foreign exchange reserves.