Russia has received new deadly ballistic missiles from Iran for use in Ukraine and is likely to use them, the US secretary of state, Antony Blinken, announced on Tuesday in London as he prepared to travel with the UK foreign secretary, David Lammy, to Kyiv. The news, confirmed by the US for the first time and seen as of huge significance to the battlefield balance ahead of Ukraine’s difficult winter, led the US and Europe to impose new sanctions on Iran, so apparently slamming the door on the prospect of a rapprochement between the new reformist Iranian government and the west. The move may also add to the pressure on the US to end its restrictions on Ukraine using British-supplied Storm Shadow missiles to strike targets deep inside Russia and not just in occupied parts of Ukraine. Ukraine, with discreet UK backing, has been pressing for a change in US policy but Blinken, at a press conference in London, highlighted obstacles to backing the Ukrainian request, including doubts about Ukraine’s ability to maintain the missiles, training and their strategic purpose. Officials ultimately said the debate turned on whether sanctioning Storm Shadow for use deep inside Russia would be seen as a dangerous escalatory step that crossed a red line set by Vladimir Putin. Blinken insisted the US position remained to provide Ukraine with the weapons it wants at the time it wants, but the Ukrainian president’s chief of staff, Andriy Yermak, said applying sanctions on Iran was not enough and Kyiv needed permission to strike deeper into Russia. The US president, Joe Biden, has allowed Ukraine to fire US-provided missiles across the border into Russia in self-defence but largely limited the distance over concerns about further escalating the conflict. The strategic decision hovering over the alliance for months is likely to be further raised in a deeper discussion due to be held between Keir Starmer and Biden in Washington on Friday, when the two leaders will assess the whole western strategy towards Ukraine, including how Kyiv can survive the winter and whether Volodymyr Zelenskiy and his new cabinet have a credible plan to defeat Russia. The German chancellor, Olaf Scholz, under domestic pressure, has called for a peace conference to be attended by Moscow, just one sign of wavering support for Ukraine in parts of Europe, but Blinken’s confirmation of Iran’s escalation may alter the debate. Germany, in a joint statement with France and the UK, condemned the Iranian decision, saying it represented a direct threat to Europe’s security by Iran and Russia. The three countries announced sanctions including banning Iranair, Iran’s civilian airline, from Europe. The missiles being provided to Russia are of the relatively new Fath-360 (BM-120) type, and not longer-range weapons. Ukraine has claimed that more than 200 Fath-360 short-range ballistic missiles were sent to a Russian Caspian Sea port. Blinken said the supply of Iranian missiles would enable Russia to use more of its arsenal for targets that were farther from the frontline in Ukraine. “This development and the growing cooperation between Russia and Iran threatens European security and demonstrates how Iran’s destabilising influence reaches far beyond the Middle East,” he said. Russia was also sharing technology with Iran, including on nuclear issues, he added. Blinken said dozens of Russian military personnel had trained in Iran in using the Fath-360 missile, which has a range of 19 to 75 miles, can carry a 150kg warhead and can be launched at a speed of Mach 3 (2,300mph – three times the speed of sound). Blinken challenged any suggestion that the new Iranian government really wanted a new relationship with the west. He said: “We’ve warned Iran privately that taking this step would constitute a dramatic escalation. Russia has now received shipments of these ballistic missiles and will likely use them within weeks in Ukraine against Ukrainians. “Iran’s new president and foreign minister have repeatedly said that they want to restore engagement with Europe. They want to receive sanctions relief. Destabilising actions like these will achieve exactly the opposite.” Faced by speculation that it had taken the decision to provide the ballistic missiles, Iran this week denied that it was providing the weapons, in a formal letter to the UN secretary general, António Guterres. “Iran considers the provision of military assistance to the parties engaged in the conflict – which leads to increased human casualties, destruction of infrastructure and a distancing from ceasefire negotiations – to be inhumane,” Iran’s mission to the United Nations said. Iran has also denied that it supplied the Russians with Shahed-136 and 131 drones. The provision of the ballistic missiles if used by Russia in Ukraine will be readily identifiable, the CIA director, William Burns, said at the weekend at an event in London. Lammy declined to comment on any internal disagreements between the UK and the US over the use of Storm Shadow, saying this was the detail of operational issues. Blinken concurred. “One of the purposes of the trip we will be taking together is to hear directly from the Ukrainian leadership including … President Zelenskiy about exactly how the Ukrainians see their needs in this moment, toward what objectives and what we can do to support those needs.” At the weekend the former defence secretary Grant Shapps, reflecting UK defence ministry thinking, said: “With the exception of our giving permission to allow UK missiles to strike Crimea, we have remained cautious about allowing our Ukrainian allies to target the source of these attacks. “The UK must issue a straightforward warning to Putin: if you continue to murder men, women and children with glide bombs launched from Russia, then we will lead the rest of the world to authorise our long-range missiles to take out your launchers.”