Iran's supreme leader on Thursday signalled openness for the first time to US offers to hold direct talks on his country's disputed nuclear drive, but voiced pessimism over the chances of a breakthrough.

Supreme leader Ayatollah Ali Khamenei has the final say in all decisions on Iran's nuclear programme which the West suspects is cover for a drive for a weapons capability.

"US officials time after time have offered one-on-one talks" on the sidelines of negotiations with the so-called P5+1 group of major powers, Khamenei said in an address broadcast live on state television.

"I am not optimistic about these (direct) talks but I am not opposed to it either," Khamenei said, while visiting the holy city of Mashhad in northeastern Iran on the occasion of Persian New Year.

The United States -- along with Britain, China, France and Russia plus Germany -- has been involved in the longrunning P5+1 negotiations with Iran.

The next round of those talks is scheduled for April 5-6 in the Kazakh city of Almaty, after Iran hailed the last round in March as a turning point.

A senior US official, speaking in Jerusalem where President Barack Obama is travelling, said that Washington was committed to the P5+1 process.

The official added: "But in that context, we would be open to bilateral discussions."

Previous calls by Washington for direct dialogue with Tehran had been shot down by Khamenei. The two governments have had no diplomatic relations for more than three decades.

Khamenei did not say what had prompted his change of heart. But he played down the prospects of a breakthrough.

"We believe the Americans are not interested in a nuclear settlement," Khamenei said, adding that US officials would "only want to dictate their own agenda" and not listen to Iranian positions.

If the US was seeking a solution, Khamenei said, "the Americans should recognise Iran's right to enrichment and then move to alleviate their concerns by implementing the regulations" of the International Atomic Energy Agency.

The IAEA, the UN's atomic energy watchdog which monitors Iran's nuclear activities, has raised the alarm over possible weapons-related work in the past.

Those allegations lie at the heart of Western suspicions about Iran's intentions.

Iran insists it regards the atomic bomb as "banned by religion," and says it operates only a peaceful nuclear energy programme.

Iran is under multiple rounds of UN sanctions over its refusal to suspend uranium enrichment, as well as additional unilateral sanctions imposed by the United States and the European Union targeting its oil income and access to the global banking system.

The US and Israel, widely believed to be the Middle East's sole but undeclared nuclear power, have also refused to rule out a military strike against Iran to prevent it getting the bomb.

Khamenei on Thursday renewed a threat of devastating retaliation against Israeli cities if Iranian facilities came under Israeli attack.

"Every now and then the leaders of the Zionist regime threaten Iran with a military attack," Khamenei said.

"They should know that if they commit such a blunder, the Islamic republic will annihilate Tel Aviv and Haifa," he said.

Iran is said to possess ballistic missiles capable of reaching Israel. It also boasts of close relations with Israel's foes in the region, including Lebanon's Hezbollah and Palestinian militants in the Islamist-ruled Gaza Strip.