With Kyiv and European capitals still stunned by Trump's surprise call with Putin, the US president also said he would "love" to have Russia back in the G7, from which it was suspended in 2014 after Moscow annexed Ukraine's Crimea peninsula.

"I think it was a mistake to throw him out," Trump told reporters in the Oval Office, referring to Putin.

Trump -- who has cast himself in his second term as a global peacemaker -- also said he would consider a summit with Putin and China's Xi Jinping "when things calm down."

"When we straighten it all out, then I want one of the first meetings I have is with President Xi of China, President Putin of Russia. And I want to say, let's cut our military budget in half."

The US president, who was hosting India's Prime Minister Nahendra Modi at the White House later Thursday, also called for the three powers to start cutting their nuclear arsenals.

"There's no reason for us to be building brand-new nuclear weapons," he added.

Trump meanwhile insisted the Russian leader wanted a ceasefire with Kyiv, despite President Volodymyr Zelensky on Thursday warning against trusting the Kremlin leader.

"I think he wants peace. I think he would tell me if he didn't," Trump said.

Trump made his comments after inking plans for sweeping "reciprocal tariffs" that could hit both allies and competitors.

- Seismic shift -

His remarks on Russia and China mark a seismic shift after more than a decade of US policy which had increasingly cast Moscow into the cold and largely viewed both it and Beijing as adversaries.

They will also be viewed with consternation by Ukraine and European allies, who will fear that if they are not at the table of international diplomacy, they could end up on the menu.

Trump's overtures to Putin in particular have caused alarm in Europe, which has viewed its huge neighbor Russia as a major threat since the invasion of Ukraine.

Trump revealed Wednesday he expected to meet Putin separately in Saudi Arabia for Ukraine peace talks, in a sudden thaw in relations.

In their first confirmed contact since Trump's return to the White House, the US president said he had held a "highly productive" conversation with his Russian counterpart who ordered the bloody 2022 invasion of Ukraine.

Several European nations have questioned Trump's strategy and warned Washington not to hatch a deal without them.

The Trump administration's talking points on Ukraine have also at times echoed Moscow's, particularly when it comes to Kyiv's dream of NATO membership to protect it from Russia.

"I believe that's the reason the war started, because (predecessor president Joe) Biden went out and said that they could join NATO," said Trump of Russia's February 2022 invasion.

In 2014, Russia was suspended from what was then the G8 after it annexed Crimea and sanctions were imposed on Moscow.

In his first term, Trump also called for Russia to be readmitted, but he found little support among other Western countries.