"Concerns of retaliation and this war escalating are huge," Kaja Kallas told reporters as EU foreign ministers met in Brussels for talks, with the Iran-Israel conflict high on the agenda.

"Especially the closing of the Strait of Hormuz by Iran is something that would be extremely dangerous and not good for anybody," she warned.

Analysts have said Iran may opt to retaliate to Washington's early Sunday attack by shutting the Strait, a waterway through which one-fifth of global oil output passes.

US Secretary of State Marco Rubio has urged China to help deter Iran from shutting down the trading route.

The European Union has been urging de-escalation since the United States joined Israel's war with Iran by striking the country's nuclear sites.

"Ministers are very much focused on the diplomatic solution," Kallas said Monday, having called on all sides at the weekend to "step back" and return to negotiations.

The EU foreign policy chief, who joined top diplomats from France, Germany and Britain for talks with Iranian Foreign Minister Abbas Araghchi in Geneva Friday, said the European outreach would continue.

"There has to be a diplomatic solution," Kallas said. "When Iran is willing to talk to us. I think we have to use this opportunity."

France, Germany and Britain called jointly on Tehran on Sunday "not to take any further action that could destabilise the region".

Speaking in Brussels, France's Foreign Minister Jean-Noel Barrot insisted Europe could bring to bear its long experience negotiating with Iran to prevent it from developing nuclear weapons.

"There is no lasting military solution to this problem," Barrot said. "Only negotiations can enable us to place long-term limits on Iran's nuclear programme."

"Europe can bring its experience, its competence, its fine knowledge of these questions to open a space for negotiations," he said.

"We reject any attempt to bring about regime change by force," Barrot added, warning it would be "illusory and dangerous" to think such a shift can be achieved with "bombs".