Donald Trump can end the Ukraine war “before the end of this year” if he carries out his threat of significant new sanctions on anyone who buys Russian oil and gas.

The prediction by Andriy Yermak, President Zelensky’s powerful top adviser, came as he insisted the US president would go through with his dramatic new plan to force Vladimir Putin to agree to peace within 50 days.

The threat of secondary sanctions would work, Yermak argued, because it would also pressure China and India — Russia’s two standout biggest fossil fuel importers — to insist on a deal so they could avoid their own new trade wars with the White House.

In an interview with The Times’s The General and The Journalist podcast, Yermak said: “Only economic problems are able to really press Putin and make him recognise that it is too expensive to him to continue this war.

“These new sanctions from the United States are so important because unfortunately all this year the price of oil has given him the opportunity to finance his military machine.

“They will create very strong pressure on Putin and he will lose a huge amount of money.

“And the countries who buy Russian oil will also press him.”

Yermak, a longstanding friend of Zelensky’s who has been his chief of staff for the past five years, added: “We will see the result before the end of this year.”

Under Trump’s plan, tariffs of 100 per cent will be levied on goods coming into the US from any country that imports any product from Russia.

Alongside Mark Rutte, Nato’s secretary-general, in the Oval Office on Monday, the US president also announced that Washington would provide Ukraine with unspecified “top-of-the-line” US weapons and Patriot air defence missiles.

Senior Russian figures have derided Trump’s threat.

Dmitry Medvedev, the former prime minister and president, and deputy chair of Russia’s Security Council, branded the package “a theatrical ultimatum” about which “Russia didn’t care”.

Dmitry Medvedev at a meeting near Moscow.

Dmitry Medvedev described Trump’s threat as a “theatrical ultimatum”

ALEXEI MAISHEV/REUTERS

Sergey Lavrov, Russia’s foreign minister, mocked Mr Trump, saying: “Fifty days. It used to be 24 hours, as well as 100 days, we’ve been through all this before”.

But Yermak told this week’s episode of the podcast he was “absolutely optimistic” that Trump would not back down.

“He knows Putin doesn’t want to stop this war now. And he knows how successful peace through strength was with Iran. That’s how he stopped the war with Iran.”

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Russia is hugely dependent on its oil and gas sales, which make up 60 per cent of all its exports and a third of all state revenue.

With most sales to Europe stopped by sanctions since the start of the full scale invasion in 2022, Moscow relies heavily on its trade with China — which buys 47 per cent of Russia’s crude oil exports — and India, which buys 38 per cent. China also takes 44 per cent of Russia’s exported coal and India buys 19 per cent of it.

Andriy Yermak and Joseph Keith Kellogg embracing at a train station in Kyiv.

Yermak greets Joseph Keith Kellogg, the United States Special Envoy for Ukraine and Russia, at a train station in Kyiv,

UKRAINIAN PRESIDENTIAL PRESS OFFICE /AP

Despite their bruising clashes earlier in the year, Trump and Zelensky are for the first time “speaking the same language” on the Ukraine war, Yermak insisted.

Ukraine’s leader has now built a strong personal bond with Trump.

“I was present in the last phone call between President Zelensky and President Trump,” Yermak explained.

“It was a great phone call. I can’t explain in a lot of detail, but I can say it was a very constructive, very productive conversation.

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“This is the great role of President Zelensky, that he is able to build incredible personal relations with the world leaders.”

Asked how difficult it was to hear the Trump administration appear to parrot Kremlin talking points on the war for months, Yermak replied: “For me, for all the people in our team, it’s not a problem if it’s necessary to have one thousand meetings to explain.

“Sometimes it’s necessary for people to understand the real situation. Of course, it’s bad that you need maybe a long time during a war, but what’s most important is the results.”

Two men participating in a podcast interview.

Yermak told the podcast “Only economic problems are able to really press Putin”

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Yermak, 53, also conceded Ukrainians were tired after three and a half years of war, and Russia’s relentless air raids on cities over the last month had made sleep difficult.

But he insisted Russia was far from breaking Ukraine’s spirit, explaining: “We can compare what happened with the Nazis bombing London. Did it break the faith and the braveness of the British people? No.

“Did it break the braveness of your leadership of Churchill and others? No.

“We have no other land, we do not have another country. We are fighting for our families, for our children”.

Yermak also suggested that China was deepening its backing of Russia in the war by encouraging its soldiers to come to fight in Ukraine as mercenaries. Some have now been taken prisoner.

He explained: “We have their documents. It’s difficult to be a mercenary in China and nobody knows about it.”

President Zelenskiy and U.S. Special Envoy Kellogg meet in Rome.

Zelensky and Yermak also met Joseph Keith Kellogg at a conference in Italy on rebuilding Uraine

REMO CASILLI/REUTERS

Yermak also called on China’s leader to become a peacemaker and agree to meet Zelensky for the first time since the full-scale invasion.

“It’s strange that President Xi never meets with President Zelensky. I think it’s time,” he added.

On the mounting number of critics in Ukraine over Yermak’s own controversial style and ever more powerful role, he insisted he welcomes criticism as “a help” if it comes from “patriots”.

But he urged his political enemies to end their attacks on him and Zelensky, arguing: “I hope that some of our politics will take the example from our soldiers, and with such pride, with such dignity, represent the country.

“We need to be united now against one enemy who is coming to our land”.

Listen to this week’s full episode of The General and The Journalist wherever you get your podcasts