The US assassination of one of Iran’s top security officials early on Friday could mean “war” with the Islamic Republic and unpredictable consequences, warned a former US intelligence officer and diplomats with expertise in the Middle East.

US officials insist the killing was a defensive measure meant to save American lives and did not signal a war. Secretary of state Mike Pompeo told US television outlets that Washington was not seeking an escalation.

But Iran is unlikely to see it that way, and the US will have to prepare for responses and consequences of an unpredictable, costly and potentially consuming conflict with the Islamic Republic.

“It’s war,” said a former US intelligence officer who has worked on Iraq and Iran. “It was always somewhat of a war but when you cut his head off, that’s war. There’s no other way around it.”

He spoke on condition of anonymity because he continues to work on sensitive security matters.

US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Show all 35 1 /35 US airstrike kills Iran's Qassem Soleimani: Fallout in pictures US airstrike kills Iran's Qassem Soleimani: Fallout in pictures This photo released by the Iraqi Prime Minister Press Office shows a burning vehicle at the Baghdad International Airport following an airstrike in Baghdad, Iraq, early Friday 3 January AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures The wreckage of the car in which general Soleimani was travelling when a targeted US airstrike struck outside Baghdad International Airport on 3 January Ahmad Al Mukhtar via Reuters US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Demonstrators burn the US and British flags during a protest in Tehran after general Soleimani was killed in a targeted airstrike by American forces Reuters US airstrike kills Iran's Qassem Soleimani: Fallout in pictures A burning vehicle at the Baghdad International Airport following an airstrike. The Pentagon said Thursday that the US military has killed general Qassem Soleimani, the head of Iran's elite Quds Force, at the direction of Donald Trump AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Protesters burn Israeli and US flags as thousands of Iranians take to the streets to mourn the death of general Soleimani at the hands of America EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Supporters of Donald Trump pray at an 'Evangelicals for Trump' campaign event held on the day following the killing of general Soleimani. At the event, the president praised the "flawless strike that eliminated the terrorist ringleader" AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures A huge procession of mourners gather in Baghdad for the funeral of general Soleimani on 4 January AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Thousands of Iranians take to the streets to mourn the death of Soleimani during an anti-US demonstration to condemn the killing of Soleimani, after Friday prayers in Tehran, Iran EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Iraqis perform a mourning prayer for slain major general Qasem Soleimani of the Iranian Revolutionary Guards at the Great Mosque of Kufa AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures A billboard reading 'Death to America and Israel', installed by Iran-backed shiite armed groups at a street in Jadriyah district in Baghdad, Iraq EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures A handout picture provided by the office of Iran's supreme leader Ayatollah Ali Khamenei shows him visiting the family of Soleiman KHAMENEI.IR/AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Thousands of Iranians take to the streets in Tehran EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Pakistani Shiite Muslims burn a mock of a US flag as they hold pictures of General Qasem Soleimani during a protest against the USA, outside the US Consulate in Lahore, Pakistan EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Iran's Ambassador to Lebanon Mohammed Jalal Feiruznia, looks to a portrait of Soleimani, as he receives condolences at the Iranian embassy, in Beirut, Lebanon AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures People make their way on the street while a screen on the wall of a cinema shows a portrait Soleimani in Tehran AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Aziz Asmar, one of two Syrian painters who completed a mural following the killing of Iranian Revolutionary Guards commander Qasem Soleimani poses next to his creation in the rebel-held Syrian town of Dana in the northwestern province of Idlib AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures A demonstration in Tehran AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures An anti-US demonstration to condemn the killing of Soleimani, after Friday prayers in Tehran EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Mujtaba al-Husseini, the representative of Iran's supreme leader Ayatollah Ali Khamenei, delivers a speech in the holy shrine city of Najaf AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Pakistani Shiite Muslims burn a mock of a US and Israeli flags as they hold pictures of General Qasem Soleimani during a protest against the USA, outside the US Consulate in Lahore, Pakistan EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Protesters demonstrate in Tehran AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Pakistani Shi'ite Muslims hold pictures of General Qasem Soleimani during a protest against the USA, in Peshawar, Pakistan EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Protesters, holding a photograph of the leader of the People's Mujahedin of Iran Massoud Rajavi, outside Downing Street in London PA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Protesters burn a US flag in Tehran AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures A Syrian man offers sweets to children to mark the killing AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Iranian worshippers attend a mourning prayer for Soleimani in Iran's capital Tehran AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Kashmiri Shiite Muslims shout anti American and anti Israel slogans during a protest AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Iranian worshipers chant slogans during Friday prayers Reuters US airstrike kills Iran's Qassem Soleimani: Fallout in pictures A protest against the USA, in Islamabad, Pakistan EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Iranians burn a US flag in Tehran EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Supporters of the National Council of Resistance of Iran in Germany (NWRI) protest outside Iran's embassy in Berlin, Germany Reuters US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Supporters of the National Council of Resistance of Iran in Germany (NWRI) protest outside Iran's embassy in Berlin Reuters US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Iranian worshippers in Tehran AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Vehicles of the United Nations Interim Forces in Lebanon (UNIFIL) patrol a road in the southern Lebanese town of Kfar Kila near the border with Israel. Following morning's killing of Major General Qasem Soleimani, Lebanon's Iran-backed Hezbollah movement called for the missile strike by Israel's closest ally, to be avenged AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Iranian women take to the streets in Tehran EPA

Diplomats and Middle East observers across the globe were stunned by the assassination, as leaders in major capitals conferred on whether the US killing amounted to the opening of a new chapter in one of the world’s most volatile regions.

“The world cannot afford another Gulf war,” Antonio Guterres, the UN secretary general, said.

Many worry it is already too late.

“We are now directly engaged in a conflict with the Iranian state,” said Dalia Dassa Kaye, director of the Middle East programme at the Rand Corporation, a research firm partly funded by the US government. “This is not just through proxies; this isn’t sanctions. This significantly increases the risk.”

Iran and the US have been locked in confrontation for 40 years, though tensions cooled as both countries found themselves facing the threat of Isis in Iraq.

The latest round of escalation began after the administration of Donald Trump pulled out of the 2015 nuclear deal forged by his predecessor and launched a campaign of “maximum pressure” targeting Iran’s economy. Washington has accused Iran of being behind a stepped-up campaign of harassment and sabotage targeting American interests. It launched airstrikes on an Iranian-allied militia on Sunday in response to a barrage of rockets that struck a base in northern Iraq, killing one American contractor.

Next came the attack on Tuesday on the US embassy in Baghdad by Iraqi militiamen loyal to Iran. No American was hurt, but protesters set the facility on fire, angering US officials who had promised a day earlier that the airstrikes on Sunday had “deterred” Iran.

The US killed Qassem Soleimani after he arrived on a plane in Baghdad and while in the company of Abu Mahdi al-Muhandis, a leader of pro-Iranian Shia militiamen.

Iran’s supreme leader Ali Khamenei, its president Hassan Rouhani and a slew of other officials vowed “vengeance” for the slaying of Soleimani, but did not hint at any specific actions.

“US needs to know that the criminal attack on General Soleimani is its biggest strategic mistake in west Asia, and that the US will not easily escape the consequences of this miscalculation,” said a statement issued by Iran’s Supreme National Security Council.

US officials have for years considered killing him, but have refrained for fear that the potential costs would outweigh the benefits.

Soleimani had long been acting with increasing impunity throughout Iraq and Syria in recent years, posting photographs of himself posing at battlefronts. One western diplomat described the killing as “quite a bold move” that “sends a message to [Iran’s] system” that the old rules of engagement no longer apply.

Still, despite his high profile, Soleimani was no swashbuckling freelancer like Osama bin Laden or guerilla leaders of the past, but an official of a government with a bureaucracy and a budget; take him out and the Iranians could replace him, which they did within hours, and continue any of his projects. Soleimani is also considered a senior Iranian official as well as a close friend of Mr Khamenei.

What are we trying to achieve here? What’s the end game? Dalia Dassa Kaye, director of the Middle East programme at the Rand Corporation

“The Quds force is a state organisation,” said the former US intelligence official. “They are part of a formal military service. They have a government and economy backing them. Is America ready to deal with that?”

Killing him risks an escalation that the US didn’t count on, and that is how big wars start, said a former US official.

“What nations know how to do is reciprocity,” said Richard Stengel, a former State Department official. “The Iranians will not see this as reciprocal. They will see this as a massively, exponentially disproportionate response which is not reciprocal or commensurate with what they’ve done.”

Mr Trump’s bluster about the killing and the open way it was conducted almost forces Iran to act.

Possible venues in which Tehran could respond to Washington include Iraq, Syria, Lebanon, and Yemen, where Iran holds significant sway and controls assets. The Islamic Republic could also launch renewed attacks against the US’s Middle East partners, including Saudi Arabia, the United Arab Emirates and Israel.

Iran has already upped pressure on its allies in Baghdad to boot US forces from the country, which it already appears inclined to do.

Iran also has cyber capabilities which it has refined in the years since the US and Israel attacked its nuclear programme with the infamous Stuxnet virus.

“They’re going to attack US interests and people,” said the former US intelligence official. “It’s not always soldiers and diplomats. There are large companies, American interests and people that can be touched. This is an old-world fight. There has to be a response or responses.”

Next week, Iran is also set to announce its pullback from the nuclear deal, which it has been chipping away at for months in incremental steps to punish western nations for abiding by US sanctions. “We have a decision due next week on whether Iran will continue to talk back from its commitments,” said Ms Kaye. “I would expect Iran to escalate its nuclear programme.”

The more important question, though, may be what the US will do after the Iranian response, and whether the US is prepared to respond to the next Iranian escalation. “What are we trying to achieve here? What’s the end game?” said Ms Kaye.

It’s unclear whether the Trump administration has thought about all of the implications of the attack, and what it means for US security policy.

“Now that we’ve made this move, we have to make counterterrorism steps and prepare for any state acts,” said the former US intelligence official.

Ms Kaye speculated that the administration was beginning to believe its own rhetoric that the regime in Tehran was on the verge of collapse due to the US sanctions, and it was convinced by hardliners in Washington that taking out Soleimani would deliver a knockout blow, or at least not cost too much.