"We call for maximum restraint, protection of civilians and full respect of international law, including the principles of the United Nations Charter, and international humanitarian law," EU foreign policy chief Kaja Kallas said in a statement.
The statement came after an emergency video conference of EU foreign ministers on Sunday, called after the United States and Israel launched military strikes on Iran, and Tehran responded with strikes on Israel, US forces and Gulf countries.
"Iran's attacks and violation of sovereignty of a number of countries in the region are inexcusable. Iran must refrain from indiscriminate military strikes," the EU statement said.
Reflecting concerns about disruptions to oil deliveries and supply chains, it said the conflict "must not lead to an escalation that could threaten the Middle East, Europe and beyond, with unpredictable consequences, also in the economic sphere".
"The disruption of critical waterways, like the Strait of Hormuz, must be avoided," the statement added.
The text was a compromise that reflected diverse views within the EU - a bloc that represents some 450 million Europeans - on the military action launched by US President Donald Trump and Israeli Prime Minister Benjamin Netanyahu.
German Chancellor Friedrich Merz said now was not the time to lecture partners and allies.
Spanish Prime Minister Pedro Sanchez, by contrast, "rejected" the US and Israeli action on Saturday, saying it "contributes to a more uncertain and hostile international order".
Behind the scenes, diplomats said Europe has little influence over the unfolding conflict, even though it may have a major impact on the continent.
"Not too many options, I am afraid. Certainly not short-term," a Western European official said.
Europeans are "just bystanders, nobody has leverage with Trump", an EU diplomat said.
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