Speaking in Kyiv alongside the UK foreign secretary, David Lammy, Blinken said the US had โfrom day oneโ been willing to adapt its policy as the situation on the battlefield in Ukraine changed.
โWe will continue to do this,โ he emphasised.
Blinken added he and Lammy would report back to their โbossesโ โ Joe Biden and Keir Starmer โ after their talks on Wednesday with the Ukrainian president, Volodymyr Zelensky.
The foreign secretary suggested Iranโs dispatch of ballistic missiles to Moscow had changed strategic thinking in London and Washington.
It was a โsignificant and dangerous escalationโ, he said.
Iranian officials say no missile was sent to Russia and the claim is a kind of psychological warfare.
Blinken added: โThe escalator here is Vladimir Putin. Putin has escalated with the shipment of missiles from Iran. We see a new axis of Russia, Iran and North Korea.โ
Lammy urged China โnot to throw in its lotโ with what he called โa group of renegadesโ.
British government sources indicated that a decision had already been made to allow Ukraine to use Storm Shadow cruise missiles on targets inside Russia, although it is not expected to be publicly announced on Friday when Starmer meets Biden in Washington DC.
The two leaders are planning to discuss the war in Ukraine, and how it could be ended, as part of a wide-ranging foreign policy discussion, though they will avoid an intense focus on any individual weapons system, as the aim of the conversation is strategic.
No press conference is scheduled during what is expected to be a short visit, Starmerโs second to the US as prime minister, after which there may be further conversations over the weekend to update key European allies on the discussions.
Wednesdayโs joint visit to Kyiv by Blinken and Lammy to meet Zelensky would not be taking place had there been no positive decision regarding Storm Shadow, the sources added.
But it would be considered unnecessarily provocative to make a public announcement about long-range missiles in Kyiv. It is also likely there will still be restrictions around Ukraineโs use of the missiles, which have a range of at least 190 miles, to avoid reckless or unnecessary attacks.
Speaking in Kyiv, Lammy said he would not reveal details of private discussions that might hand Putin an advantage. He denounced the Russian leaderโs โsinisterโ invasion of Ukraine and accused him personally of โarrogance and greedโ.
โThis is imperialism. This is fascism,โ he added.
Ukraine has been lobbying for months for permission to hit airfields, missile launchers and command and control centres deep inside Russian territory. Speaking to the Guardian in May, Zelensky said Bidenโs equivocation and incremental approach had cost lives. It allowed the Kremlin to โhuntโ Ukrainians, he complained.
Zelensky urged the president to overcome his perennial worries about nuclear escalation. The US should โbelieve in us moreโ, he stressed, saying of Russia: โWe have to respond. They donโt understand anything but force.โ
Blinken and Lammy arrived in Kyiv on an overnight train from Poland. They had come to reiterate โironclad support for Ukraineโ, Lammy posted on X.
โWe must stand up to Vladimir Putinโs imperialism. Our collective security depends on it.โ
Russiaโs attacks on civilians were โhorrific, barbaric, unbelievableโ, he commented.
The trip came 24 hours after Blinken confirmed in London that Tehran had shipped new deadly ballistic missiles to Russia. In Kyiv, Blinken stated he was delivering a strong message that Britain and the US were committed to Ukraineโs โsuccessโ and โvictoryโ.
Speaking at a joint press conference, Lammy described the UKโs backing for Ukraine as an enduring โhundred-year partnershipโ. He added the government was providing a new package of military assistance including Brimstone missiles and AS-90 self-propelled guns.
โWe recognise what is at stake: not just the liberty of Ukraine but the security of Europe and the west,โ Lammy declared.
There was growing anticipation in Kyiv that the US and UK would finally lift their objections to long-range strikes.
Ukrainian officials had previously expressed frustration that the new Labour government in the UK had not been more robust on the issue, and had waited for the White House to amend its red lines. The US state department has reportedly been open to Kyivโs request, with the Pentagon and some in the US intelligence community sceptical.
In May the US allowed weapons such as Himars artillery to be used within Russiaโs border regions for the first time. This followed a Kremlin offensive into the Kharkiv region and the Ukrainian city of Vovchansk.
This week senior Democrats and Republicans in Washington urged the White House to go further. In a letter, a group of senators called on Biden โto immediately endโ his administrationโs โlimitationsโ on the use of long-range missiles provided by the US and its NATO allies.