US Secretary of State Mike Pompeo says US is prepared to engage in a conversation. (File)

US Secretary of State Mike Pompeo said Sunday his country was ready to talk with Tehran "with no preconditions", but there was no indication if lifting sanctions over Iran's nuclear programme was on the table.

The top US diplomat, who is considered a hawk on the Iran file, appeared to soften the US stance somewhat following weeks of escalating tensions with Tehran.

"We are prepared to engage in a conversation with no preconditions," Pompeo said in Switzerland, which in the absence of US-Iranian diplomatic ties represents Washington's interests in the Islamic Republic.

"We are ready to sit down with them," Pompeo told a joint news conference with his Swiss counterpart Ignazio Cassis at the impressive medieval Castelgrande castle in Bellinzona, nestled in the Alps in Switzerland's Italian-speaking Ticino region.

He was reacting to comments made by Iranian President Hassan Rouhani on Saturday insisting that his country would not be "bullied" into talks with the United States, and that any dialogue between the two countries needed to be grounded in "respect".

Iranian Foreign Minister Mohammad Javad Zarif also said in an interview with the American network ABC broadcast Sunday that it was "not very likely" that Tehran would agree to talks with the US any time soon.

US President Donald Trump, he said, "is imposing pressure."

"This may work in a real estate market. It does not work in dealing with Iran," he said, insisting that "threats against Iran never work... Try respect. That may work."

Pompeo himself also appeared to back-pedal on the offer to have condition-free talks with Iran, stating that Washington was "certainly prepared to have (a) conversation when the Iranians will prove they are behaving as a normal nation."

"Malign activity"

Nonetheless, Pompeo's comments mark the first time the Trump administration has offered no-strings-attached talks since the recent escalation began in the wake of the US withdrawal from a hard-won 2015 nuclear deal between Tehran and world powers.

But Pompeo stressed that "the American effort to fundamentally reverse the malign activity of this Islamic Republic, this revolutionary force, is going to continue."

In other words, Washington has no intention to let up on its campaign of "maximum pressure" on Iran.

Pompeo himself last year laid out 12 draconian demands he said Iran would need to meet before reaching a "new deal" with the United States, essentially addressing every aspect of Iran's missile programme and what Washington calls its "malign influence" across the region.

Washington has since reimposed sanctions, and has been locked in an increasingly tense standoff with Tehran.

Last month it deployed an aircraft carrier task force, B-52 bombers and an amphibious assault ship to the Gulf, along with additional troops against what Washington's leaders believed was an imminent Iranian plan to attack US assets.

But at the same time, Trump has over the past week toned down the rhetoric, saying Washington does not seek "regime change" in Iran and holding out the possibility of talks.

He said the US was merely "looking for no nuclear weapons," adding that "I really believe that Iran would like to make a deal. I think that's very smart of them and I think there's a possibility for that to happen also."

Swiss mediation?

Swiss Foreign Minister Cassis meanwhile voiced his country's readiness to play the role of "intermediary" between the two countries.

But he stressed Switzerland could not be "mediators if there is not willingness on both sides."

Cassis also voiced concern about the "great suffering" in Iran brought about by the US sanctions, and urged Washington to identify a financial "channel" to allow the Iranians to purchase humanitarian aid without being slapped with US punitive measures.

Pompeo did not respond directly to this request, but he rejected the notion that US sanctions were causing suffering, instead blaming the leadership in Tehran.

The challenges facing Iranians "are not caused by our economic sanctions," he said. "They're caused by 40 years of the Islamic regime not taking care of their people and instead using their resources to destroy lives."

He meanwhile preferred to remain discreet about efforts, largely led by Switzerland, to ensure the release of a handful of American citizens being held in Iran, stating only that the issue was a top priority for Trump, and that Washington is "working with all willing nations to assist us."