The EU's diplomatic chief said Thursday the bloc would support talks between the United States and Iran, but only if the current nuclear deal with Tehran is preserved.

Washington and Tehran have been locked in a bitter standoff since last year when US President Donald Trump unilaterally pulled out of the 2015 deal that gave Iran relief from sanctions in return for curbs on its atomic programme.

Tensions have risen dramatically in the Gulf, where Iran has seized tankers, but EU countries are reluctant to join a US-led operation to protect commercial shipping.

Instead, EU foreign and defence ministers meeting in Helsinki discussed the possibility of launching their own observation mission in the Strait of Hormuz, a strategically important choke point at the entrance to the Gulf.

The idea of direct talks between Washington and Tehran as a way out of the crisis grew this week after Trump mooted the idea and the new US defence secretary urged Iran's leaders to engage.

The European Union has desperately sought to prevent the deal from collapsing completely, arguing it is the best way to stop Iran developing nuclear bombs.

EU diplomatic chief Federica Mogherini gave a cautious welcome to the idea of negotiations, after Trump said Monday he was ready to meet Iran's President Hassan Rouhani within weeks.

"We are always in favour of talks, the more people talk, the more people understand each other better, on the basis of clarity and on the basis of respect," Mogherini said as she arrived for the Helsinki meeting.

But she added that "first and foremost what is existing needs to be preserved" -- referring to UN Security Council resolutions and specifically the 2015 deal known formally as the Joint Comprehensive Plan of Action or JCPOA.

"We will always advocate for the full respect by all sides of the UNSC resolutions and that includes the JCPOA," she said.

At the recent G7 summit in Biarritz, Trump showed openness to French President Emmanuel Macron's proposal of a summit with Rouhani.

US Defence Secretary Mark Esper followed up on Wednesday by urging Tehran to negotiate, but Iranian Foreign Minister Mohammad Javad Zarif insisted Washington must respect the deal and halt what he called "economic terrorism" against his country.

German Foreign Minister Heiko Maas welcomed the "greater willingess for dialogue" seen since the G7 and urged Iran to engage.

"It is now a matter of operationalising this and ensuring that everyone makes a contribution -- including Iran -- which leads to de-escalation in the region," Maas said.

Maritime security and the Middle East are on the agenda in Helsinki, but apart from Britain, there has been little European enthusiasm for Washington's Operation Sentinel in the Gulf.

Esper said the effort to protect shipping on crucial oil trade routes was now "up and running" with help from Britain, Australia and Bahrain.

French Defence Minister Florence Parly said that up to five countries had signed up for a potential EU observation mission.

"We must guarantee free navigation and security in the Strait of Hormuz, a vital region for the transport of hydrocarbons," she told AFP.

"But there is no question of creating an escort force -- rather a dissuasive presence with an exchange of information." France, which already has a warship in the region, is keen to avoid any perception that it is aligning itself to Trump's strategy of "maximum pressure" on Iran.

A report by the German Council on Foreign Relations think tank assessing the options for an observer mission suggested it could require significant resources: five frigates or destroyers with helicopters, three maritime reconnaissance aircraft and at least one supply ship.

Iran has said that sending a "European fleet" to the Gulf would be a provocative move and other EU countries struck a more cautious note.

"If we speak about whatever operation there we need to be careful to avoid any kind of military escalation," said Edgars Rinkevics, foreign minister of Latvia, which currently has one of its seamen in Iranian custody.

"In my own country we are very far away from any kind of position that would be supportive of any kind of engagement there."

In response to the US pulling out of the deal and reimposing sanctions, Tehran has breached certain limits on its nuclear production imposed by the accord.

An EU official said the focus now was on ensuring these breaches do not become irreversible.

An Iranian delegation is expected in Paris next week and a meeting between senior Iranian and EU officials is planned soon.