Iran has taunted the US with chilling billboards that have fueled conspiracy theories about Senator Lindsey Graham's death and hinted at a possible assassination plot against President Donald Trump. 

A massive red billboard popped up in Valiasr Square in Tehran this week, with an eerie message reading: 'Who is D nexT one? #lindseygraham'. 

It appeared to hint that Trump, 80, is Tehran's next target, as the slogan in English had the unusual capitalisation of the letters 'D' and 'T' appearing to reference the 47th US president.

In Enghelab Square, in central Tehran, a gigantic corner billboard displayed an image of the US president, his body dressed in a suit peeking out from an open black coffin. Written underneath, in graffiti-style in Persian and English, were the words: 'We will kill Trump'.

These political billboards are the latest in a series targeting the US president. Earlier this week, a mural with individual, framed images of Trump, his children and Melania set above US flag-draped coffins and a backdrop of the White House, was unveiled in Palestine Square in Iran. The timing coincided with American missiles attacking bridges, railways and water facilities in Iran.  

The newest billboards have also fueled conspiracy theories that the Middle Eastern country may have been involved in Graham's mysterious death at the age of 71 last weekend, as the politician was in Ukraine just days before, leading people to speculate Tehran may have orchestrated it. 

There has been no confirmation or evidence suggesting Iran had anything to do with the senator's death.  

A day after returning from Ukraine and just hours before his death, the South Carolina senator spoke to Trump on the telephone. 

An unidentified person, who spoke with Graham in the moments after his conversation with the President, told news outlet Axios that he complained of feeling unwell.

When the individual urged Graham to seek medical attention immediately, the senator said he would do so on Sunday morning after his scheduled appearance on NBC's Meet The Press.

He then joked: 'I can't die now. I still need to do the Russian sanctions, get Iran sorted out, and do Israeli-Saudi normalisation,' according to the outlet.

But just a few hours after making the quip on July 11, the senator was pictured being wheeled out of his home on a gurney as first responders attempted life-saving measures in photos obtained by TMZ.

He appeared to be intubated as EMTs lifted the gurney into an ambulance that then rushed to the George Washington University Hospital.

Authorities first received a 911 call about a 'cardiac arrest' at around 8.30pm that night, according to multiple reports. A large number of first responders were then seen descending on the street outside Graham's house.

Just about 25 minutes after the call, first responders had started CPR as the senator appeared to have had a cardiac arrest, according to police scanner audio obtained by The Washington Post.

Early on July 12, Graham's office announced he had 'passed away from a brief and sudden illness'.

His cause of death has since been revealed to be 'aortic dissection due to arteriosclerotic cardiovascular disease'.

The Mayo Clinic defines his as a tear in the inner layer of the aorta, the body's main artery. It can be fatal if blood rushes through the tear. In Graham's case, plaque buildup on his heart weakened the aortic walls, causing a tear. 

After the shocking Iranian billboards and Graham's death, Trump took to his social media platform Truth Social early Sunday morning to call for additional measures to be taken against Iran in the senator's name. 

'Republicans should add Iran to the Russian Sanctions Bill. That’s what Lindsey wanted to do, and it was going to happen. IMPORTANT!!!' he wrote. 

The bill, which was the brainchild of Lindsey Graham, aims to impose mandatory sanctions on Russia and its allies. and potentially broaden the reach of penalties to include tariffs on buyers of Russian energy.

It is unclear who is behind the Iranian billboards. The Daily Mail has reached out to the White House for comment. 

Iranian officials are now urging people to stay away from US military bases in the Middle East. 

'In response to US acts of aggression and threats from terrorist groups, Iran has launched missile strikes targeting Sulaymaniyah (Iraq) and Bahrain,' the Islamic Republic of Iran Broadcasting said on X. 

'People should stay away from US bases.' 

Iran's new leader, Ayatollah Mojtaba Khamenei, took to X to call out Trump, writing: 'Now that the American enemy seeks to ignite war and endure heavier costs, it should know that the Iranian nation and the Resistance Front have unforgettable lessons in store for it.' 

Khamenei also called Trump the 'Great Satan'. 

'Today, the Great Satan has once again revealed its true, unmasked face, so that this dark episode of criminality and broken promises serves as yet another indisputable testament to the US’s dishonesty, irrationality, unreliability, and malicious nature,' the leader wrote on X. 

'Coercion, totalitarianism and brutality are inseparable components of the US creed and doctrine.

'The repeated breaches of the agreement by the Great Satan [the US] regarding the MOU signed by the Presidents of Iran and the US have once again laid bare a fundamental truth: the signature of the US President is utterly worthless and devoid of credibility.' 

Speaking to Iranians, Khamenei said the government would seek to avenge citizens and that 'vengeance is what our nation is demanding, and this must definitely be done'.