Fears have mounted over the world’s two major wars converging after Ukraine attacked an Iranian vessel in the Caspian Sea, with Tehran vowing to respond.

Iran claimed one sailor was killed and another injured in the Ukrainian attack, as the ministry summoned Ukraine’s chargé d’affaires in Tehran to protest against what it described as a “hostile and criminal” act.

Foreign minister Abbas Araghchi said on Sunday the reported strike on a commercial vessel “cannot go unanswered” in a call with the EU’s top diplomat, Kaja Kallas, and his Russian counterpart, Sergei Lavrov.

Why is the Caspian Sea important?

The Caspian Sea is bordered by five countries, including Iran and Russia.

With the blockade of Iranian ports in the Persian Gulf and the Gulf of Oman, the Caspian has taken on new significance as a trade route.

Why is Ukraine attacking ships in the Caspian Sea?

Ukraine officials have claimed the Iranian vessel was carrying components for drones and missiles from Iran to Russia, but Tehran claimed the vessel was “entirely civilian” and was carrying “no military cargo”.

Iran is a major supplier of weapons to Russia with its Shahed drones. In August 2024, Ukraine attacked a cargo vessel in the Caspian Sea, destroying a ship reportedly loaded with Shahed-136 drone components and Iranian munitions.

But former British army officer and weapons expert Hamish de Bretton-Gordon believes that Ukraine’s most recent attack on the Caspian Sea is a “message to Trump” first and foremost.

He argued that by executing an attack around 500 miles away from Kyiv, Ukraine is demonstrating its capability and showing the US president how it can help take on Iran.

He told The Independent: “Ukraine is trying to show Trump that the Russians are helping Iran out. Ukraine is demonstrating that they can prevent that and help with Trump’s Iran problem, but the US needs to help Ukraine in return.”

It comes as Volodymyr Zelensky accused Russia of providing Iran with satellite observations of the Gulf States and US military sites in the Middle East to facilitate strikes there, and offered the US president access to this intelligence, in a move that could strengthen the view that Ukraine is trying to gain more US support against Russia.

But Natia Seskuria​​​​, a senior fellow in Russian and Eurasian security, believes Ukraine’s strike in the Caspian Sea is “in line with the country’s strategy of targeting Russian military capabilities” rather than “opening a new theatre against Iran”.

She said: “The key strategic objective is to disrupt the flow of Iranian military support to Russia and create additional pressure on the logistical networks that enable Moscow to sustain its military campaign.”

Following Mr Zelensky’s meeting with Donald Trump in Washington this week, it was announced that the US will give Ukraine a licence to make Patriot defence systems, granting a long-running request from Kyiv to counter Russian missile attacks.

How could Iran attack Ukraine?

Iran’s ambassador to Russia, Kazem Jalali, has called on Russia to respond to the strike, according to Iranian state media. He reportedly said Russia is a “friendly nation” and that the attack deserved a “serious response”.

However De Bretton-Gordon said there is no “strategic advantage” to Iran attacking Ukraine, and it would be done with “a lot of difficulty”.

Dr Sidharth Kaushal, a senior research fellow of Military Sciences at Royal United Services Institute, said: “Quite a few Iranian ballistic missiles can hit at least parts of Ukraine, the Shahab 3 and Khorramshahr 4, for example.

“Some ballistic missiles with shorter advertised ranges might also cover the distance with a lighter warhead. Iran lacks a cruise missile capable of covering these distances, which would also be a bit long for Shahed-type uavs (unmanned aerial vehicles).”

However, he added there would be little benefit in Iran actually responding to Ukraine with aggression, as he said: “I doubt the Iranians are overly keen to expend missiles from a part of their arsenal that is important, relatively numerically constrained, and difficult to replace while widening their list of adversaries in the process.

“My guess is that the Iranians will shrug off what happened in the Caspian Sea as the cost of doing business.”