*Iran insists on revenge for Gen Soleimani's killing
Nato called on Tehran to "refrain from further violence and provocations" after Iran threatened retaliation to the US assassination of one of its top military commanders, as European capitals grapple with the fallout from the incident.
The United States airstrike that killed Iran's elite Quds Force leader Qassem Soleimani in Baghdad has prompted fears over an outbreak of war in the region and left the nuclear agreement struck with Tehran hanging by a thread.
"New conflict would be in no one's interest," Nato Secretary-General Jens Stoltenberg said in Brussels Monday, after a meeting of national ambassadors to the defence alliance.
"For years all [Nato] allies have expressed concern about Iran's destabilising activities in the wider Middle East region," Stoltenberg added.
The US briefed its Nato allies on recent incidents in the Gulf, according to the military alliance's head, and on the Soleimani strike.
Asked whether he understood the US move — which has unleashed rage and massive protests in both Iraq and Iran — Stoltenberg said it was a "US decision" and not one taken by the global coalition against the Islamic State or Nato.
United Nations Secretary-General Antonio Guterres urged global leaders Monday to "stop escalation," warning that geopolitical tensions were at their highest level of the century.
"The New Year has begun with our world in turmoil," Guterres said in brief remarks at the UN headquarters in New York.
"We are living in dangerous times."
German Chancellor Angela Merkel, French President Emmanuel Macron and British Prime Minister Boris Johnson also called for restraint in the Gulf, in a statement released late Sunday.
The US strike has also ushered in the apparent demise of the 2015 nuclear accord aiming to curb Iran's ability to build nuclear weapons, a development European leaders had been been striving to avoid.
Tehran said on Sunday that it no longer sees itself as bound by the deal, though observers believe it has left open a back door to return to compliance with the agreement.
The EU is now awaiting the assessment of the International Atomic Energy Agency, which monitors the situation on the ground in Iran,
before deciding its next move, spokesman Peter Stano added.
EU foreign ministers are to hold an extraordinary meeting in Brussels on Friday in light of the situation in the Middle East.
European Commission President Ursula von der Leyen pledged to activate all the EU's diplomatic channels in the Middle East to help de-escalate tensions between the US and Iran.
Meanwhile, France's foreign minister said Tehran must not retaliate over the killing of top general Qasem Soleimani, amid an escalating war of words between Iranian officials and President Donald Trump.
"It is essential that Iran renounce any reprisals or retaliations," Jean-Yves Le Drian told BFM television in Paris, adding that "there is still a place for diplomacy, fortunately."
"In all the talks I've held with other officials, no one wants a war," he added.
Le Drian said any counterstrikes against the US would jeopardise the viability of the coalition fighting the Islamic State group threat in Iraq and Syria, which he said must remain the region's priority.
Reuters adds: Iran's supreme leader wept in grief with hundreds of thousands of mourners thronging Tehran's streets Monday for the funeral of Soleimani, killed by a US drone on the orders of US President Donald Trump.
As the coffins of General Qassem Soleimani and Iraqi militia leader Abu Mahdi al-Muhandis, who also died in Friday's attack in Baghdad, were passed over the heads of mourners, Soleimani's successor vowed to expel US forces from the region in revenge.
The killing of the 62-year-old Soleimani, architect of Iran's drive to extend its influence across the Middle East, has stoked concern around the world that a broader regional conflict could erupt.
Trump has listed 52 Iranian targets, including cultural sites, that could be hit if Iran retaliates with attacks on Americans or US assets, although US officials sought to play down the president's reference to cultural targets.
"Never threaten the Iranian nation," Iranian President Hassan Rouhani tweeted in response to Trump's threat.