Iran had long warned that if Washington attacked, it would target its regional assets -- which in principle should include NATO-member Turkey where US troops are stationed at several bases.

Since the conflict began early on Saturday, Iran has fired missiles and drones across the Middle East, striking Israel, and at Iraq, Jordan and all six Gulf states in a bid to strike at US assets.

But not Turkey, despite the presence of several highly sensitive sites hosting an unspecified number of US troops.

One is Incirlik, a key NATO air base used for decades by US troops near the southern city of Adana, another is Kurecik, a base in central Turkey which has a NATO early-warning radar system that can detect Iranian missile launches.

Although Ankara has categorically denied the radar data has ever been used to help Israel, its presence has rattled Tehran.

"In the past, Iranian officials have pointed to Kurecik to signal displeasure over the radar installations but.. at this point, attacking a NATO country like Turkey would be an even riskier gamble for Iran," said Gonul Tol of the Washington-based Middle East Institute.

The bases are a deeply sensitive issue for Turkey with police on Saturday arresting three journalists for "national security offences" over footage filmed near Incirlik shortly after the Iran strikes began.

On Monday, Ankara roundly denied unsubstantiated online rumours alleging "a US military base in Turkey has been hit", saying there were no "foreign" owned bases and "no attack against our country".

- 'Extremely high risk' -

Unlike its strikes on the Gulf countries, an attack on Turkey would not be a low?risk "messaging operation" but "a high?cost strategic gamble for Iran," said Arif Keskin, an Iranian expert at Ankara University.

"A direct military move against Turkey would risk triggering a symmetrical response from Ankara. This could push the conflict beyond manageable limits," he told AFP.

Attacking a NATO member could also trigger the military alliance's collective defence mechanisms, which would "raise the strategic cost dramatically," he said.

Serhan Afacan, director of the Ankara-based Centre for Iranian Studies (IRAM), said Iran appeared to have banked on the fact none of the Gulf states would retaliate -- which wouldn't be the case with Turkey.

"Iran has neither the strategic incentive nor the intention to target anywhere in Turkey. The risks of such a move would be extremely high for Tehran, both politically and militarily," he said.

It would also risk isolating one of its last potential avenues for negotiation.

"Iran continues to value Turkey's potential role in de-escalation and diplomatic mediation. Targeting Turkey would undermine that channel at a moment when dialogue remains crucial," he told AFP.

- Important neighbour -

Turkey had furiously worked diplomatic back-channels to head off the military confrontation, but since Saturday has sought to maintain a neutral stance.

President Recep Tayyip Erdogan said Saturday he was "deeply disturbed" by the attacks while also denouncing Iran's retaliatory strikes, vowing to step up diplomatic efforts to bring the sides back to the negotiating table.

For Tehran, Ankara remained "a critical diplomatic actor" who could play a key in back-channel diplomacy, Keskin said.

"An attack on Turkey would narrow Iran's diplomatic manoeuvring space and cut off a crisis?management route it may later need."

It also risked pushing Ankara into "an opposing camp" which could play out badly for Tehran given Turkey's geostrategic location, he said.

"Iran's choice not to attack Turkey is not a matter of goodwill but the outcome of a highly layered strategic calculation," he said.