Number 10 has rejected calls from a prominent US politician that Vladimir Putin should be assassinated.
The comments come after Lindsey Graham, a senior Republican senator for South Carolina, called for somebody in Russia to "take out" Putin to bring the invasion of Ukraine to an end.
He made the remarks following reports of a fire at Europe's largest nuclear power plant in Zaporizhzhia after Russian troops attacked Ukrainian forces in the area, sparking panic of a nuclear disaster.
The fire has since been extinguished, with Russian soldiers taking control of the power plant and experts claiming there is no evidence of an increase in radiation following the blaze.
"Is there a Brutus in Russia? Is there a more successful Colonel Stauffenberg in the Russian military," he said on Twitter, referencing the army officer who unsuccessfully attempted to kill Adolf Hitler.
"The only way this ends is for somebody in Russia to take this guy out. You would be doing your country - and the world - a great service.
"The only people who can fix this are the Russian people. Easy to say, hard to do.
"Unless you want to live in darkness for the rest of your life, be isolated from the rest of the world in abject poverty, and live in darkness you need to step up to the plate."
When asked whether the prime minister agreed with Graham's comments, a spokesperson for Boris Johnson firmly rejected the calls for the Russian president's assassination.
“No, we stand with the Ukrainian people in demanding the immediate end to the Russian invasion," he said.
“We’ve said before that Putin must be held [to] account in front of an international court for the horrific act he’s committed.”
Russia has responded to Graham's comments with outrage, claiming the remarks are an example of 'Russophobia'.
Kremlin spokesperson Dmitry Peskov told reporters on Friday: "Of course, these days not everyone is managing to preserve a sober mind, I would even say a sound mind."
He also called for national unity from Russians: "Now is not the time to divide, now is the time for all to unite, be together, and unite of course around our president."
The Russian ambassador to the US, Anatoly Antonov, branded the statement "unacceptable and outrageous".
"The degree of Russophobia and hatred in the US towards Russia is off the charts," he said.
"It's unbelievable that a senator of a country that promotes its moral values as a 'guiding star' for all mankind could afford to call for terrorism as a way to achieve Washington's goals in the international arena.
"It's getting scary for the fate of the United States, which has such irresponsible and unprofessional politicians at the helm."
The remarks come after an escalating war of words between the US and Russia in recent days.
On Thursday, Russian foreign minister Sergei Lavrov likened the US to Napoleon and Hitler and suggested they controlled Europe.
"Napoleon and Hitler, they had the objective of having the whole of Europe under their control," said Lavrov.
"Now Americans have Europe under their control.
"We see that the situation has really demonstrated what role the EU is playing in the context of the global situation – they're just fulfilling a role."
On Monday, the US accused Russia of a "totally unacceptable" escalation after the Russian president announced he had put the country's nuclear deterrent on standby because of "aggressive statements" about Russia from Nato.
“Senior officials of the leading Nato countries also allow aggressive statements against our country," said Putin in a televised address, announcing the decision.
"Therefore, I order the minister of defence and the chief of the general staff to transfer the deterrence forces of the Russian army to a special mode of combat duty."
The West is desperate to avoid escalating the situation into all out war with Russia, a sentiment which was reemphasised today by Nato secretary-general, Jens Stoltenberg.
"We are not part of this conflict, and we have a responsibility to ensure it does not escalate and spread beyond Ukraine," he said.
Nato secretary-general Jens Stoltenberg has said Russia is using cluster bombs in Ukraine. “We have seen the use of cluster bombs and we have seen reports of use of other types of weapons which would be in violation of international law,” he told reporters in Brussels. Stoltenberg further warned that “the worst is yet to come” in the conflict.
Ukraine’s demands for a no-fly zone were once again rejected by Nato allies, citing fears the move could spiral into all-out war between Russia and the West. Ukrainian president Volodymyr Zelenskyy said that if allies wouldn’t meet his request to protect his nation’s air space, they should instead provide Kyiv with more war planes.
A huge fire was put out at a nuclear power plant in southeast Ukraine after it was seized by Russian forces. No damage was done to reactors at the Zaporozhzhia plant after a projectile hit a nearby building, UN nuclear chief Rafael Grossi said. Boris Johnson accused Putin of “directly threatening the safety of all of Europe” after the attack on the plant.
Russia’s parliament passed a low imposing jail sentences of up to 15 years for people who spread “fake information” about the country’s armed forces. The laws give the Russian state enhanced powers to crack down on dissenters. Russia’s communications watchdog also restricted access to several foreign news organisations’ websites, including the BBC and Deutsche Welle.
Watch: Lindsey Graham calls for assassination of Russian president Vladimir Putin