French President Emmanuel Macron called for an emergency UN Security Council meeting in response to the US and Israeli strikes in Iran.
Germany and the UK are holding their own emergency meetings on Saturday to discuss the situation.
The European Union is evacuating some staff from the region.
The responses come after the US and Israel launched a major attack on targets across Iran, and US President Donald Trump called on the Iranian people to "take over your government" — an extraordinary appeal that suggested they could be seeking to end the country's theocracy after decades of tensions.
The strikes by the US create a dilemma for its democratic allies.
While European leaders firmly oppose Iran's nuclear program and crackdowns by its hard-line theocracy, they are loath to embrace unilateral military action by Trump that could breach international law and unleash a broader conflict.
Trump's strikes on Iran last June and the arrest of Venezuela's Nicolas Maduro last month caused a similar quandary.
It was unclear whether US allies were given any advanced warning of the attacks.
The German government said it was only given notice Saturday morning.
France's junior defence minister said France knew something would happen, but didn't know when.
"The escalation underway is dangerous for everyone. It must stop," Macron said in a statement.
France, which has military presence in the United Arab Emirates, Qatar and Jordan, would offer military aid to its partners in the Middle East, the president said.
"The outbreak of war among the United States, Israel and Iran has serious consequences for peace and international security," Macron said.
He called on Iran's leadership to commit to negotiations on its nuclear and ballistic programs.
"The Iranian people should also be able to build their future freely. The massacres perpetrated by the Islamic regime disqualify it, and necessitate that the people be given a voice," he said.
British Prime Minister Keir Starmer chaired a meeting of the government's emergency committee on Saturday morning.
"We do not want to see further escalation into a wider regional conflict," a UK government spokesperson said, reiterating Britain's support for a negotiated solution to Iran's nuclear ambitions. Britain was not involved in the strikes.
Responding to the attack, the European Union's top diplomat called the conflict in the Middle East "perilous" and said she was working with Israeli and Arab officials to pursue a negotiated peace.
"Iran's regime has killed thousands. Its ballistic missile and nuclear programs, along with support for terror groups, pose a serious threat to global security," said Kaja Kallas, foreign policy chief of the 27-nation bloc, in a post on social media.
The Nobel Peace Prize-winning International Campaign to Abolish Nuclear Weapons condemned the US and Israeli strikes on Iran in harsher words.
"These attacks are totally irresponsible and risk provoking further escalation as well as increasing the danger of nuclear proliferation and the use of nuclear weapons," said its executive director, Melissa Parke.
Malaysian Prime Minister Anwar Ibrahim condemned Israeli strikes on Iran and accompanying US military action, warning that the escalating conflict has pushed the Middle East to the "edge of catastrophe."
