The German Foreign Minister increased pressure on Iran on Wednesday with a warning that conflict in the Arabian Gulf could not be ruled out after attacks on land and sea.
Heiko Maas told his French Foreign Minister Jean-Yves Le Drian that "the situation is serious".
Mr Le Drian outlined his concerns about Tehran's words and action, and said there was little time to launch diplomatic initiatives that would ease the tension.
The remarks came a day after German Chancellor Angela Merkel said there was “strong evidence” that Iran was behind an attack on two oil tankers last week in the Gulf of Oman.
Mrs Merkel joined the growing chorus of western leaders to point the finger at Tehran for the attacks.
“We of course take these explanations very seriously and there is strong evidence,” she said of the US accusations.
“But that does not prevent me from saying that we must do everything to solve the conflict with Iran in a peaceful way.
"We will do everything to impress on all sides, but especially to make clear to Iran, that this serious situation mustn't be aggravated."
On Wednesday, Iran repeated its threat to expand its nuclear capacity, breaking the limits set in a deal agreed to with western powers in 2015.
Last week, the US released footage it said showed members of Iran's Islamic Revolutionary Guard Corps removing an unexploded limpet mine from one of the tankers.
On Wednesday the French and German foreign ministers held talks in an effort to try and prevent more conflict breaking out.
"There is still time and we hope all the actors show more calm. There is still time, but only a little time," Mr Le Drian said.
Iran recently threatened to up its enrichment of low-grade uranium passed the limit it is allowed under the agreement it signed in 2015.
It believes it is entitled to do so because the US pulled out of the pact and renewed sanctions against Tehran, which has given European powers until July 8 to find a way to protect the Iranian economy.
"What we are doing, despite some countries' propaganda against it, is the minimum measure Iran can take,” Iranian President Hassan Rouhani said.
“The basis and the spirit of the deal have been seriously damaged by other parties."
Iran's atomic agency spokesman, Behrouz Kamalvandi, on Wednesday told Tasnim news agency: "Iran's two-month deadline to remaining signatories cannot be extended, and the second phase will be implemented exactly as planned."
Mrs Merkel warned on Tuesday that Tehran would do this.
"We anticipate that Iran will continue to uphold the deal," she said. "When that isn't the case, there will naturally be consequences."
The EU has tried, seemingly in vain, to keep the deal alive despite the increasing tension between Iran and the US.
“It is not an easy exercise,” the EU’s foreign policy chief, Federica Mogherini, said on Monday.
"During the past year, it has become increasingly difficult for all to keep the nuclear deal fully implemented."