US President Donald Trump criticized both sides in the Russia-Ukraine conflict, expressing his frustration as efforts to resume ceasefire talks continue to falter.

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Initially, Trump said he was "very angry" with Russian President Vladimir Putin, threatening to impose new tariffs on Russian oil.

NBC anchor Kristen Welker said the US president called her to express his anger over Putin's questioning of Ukrainian President Volodymyr Zelensky's future as leader.

During her program "Meet the Press" on Sunday, Welker reported on comments made by Trump in an early morning phone call.

He was quoted as warning that "if Russia and I cannot reach an agreement to stop the bloodshed in Ukraine, and if I determine that this was Russia's fault, I will impose secondary tariffs on oil coming out of Russia."

Welker noted that Trump told her he was "very angry and upset" when Putin began making comments about Zelensky's credibility and talking about new leadership in Ukraine.

He indicated that he would speak to the Russian president next week.

Trump later softened his tone toward Putin and directed his anger at Zelensky, warning him of problems if he backed out of a deal granting the United States mining rights in Ukraine.

Since taking office, the US president has sought a swift end to the more than three-year-old war in Ukraine, but his administration has failed to achieve a breakthrough.

Putin rejected a joint US-Ukrainian plan for a 30-day ceasefire and proposed on Friday that Zelensky step down as part of the peace process, reflecting a more hardened Moscow negotiating position and angering Kyiv.

Trump told NBC that Putin knows he's angry, noting at the same time that he "has a very good relationship with him," and explaining that "anger dissipates quickly... if he does the right thing."

Trump spoke to reporters aboard Air Force One as he returned to Washington from his home in Mar-a-Lago, Florida, softening some of his criticism.

"I was disappointed in a way," Trump said, expressing his irritation at Putin's questioning of Zelensky's credibility, because "he's supposed to make a deal with him, whether you like it or not."

"So I wasn't happy about that, but I think it would be good, and I certainly wouldn't want to impose secondary tariffs on Russia," he added.

Trump then moved on to criticize Zelensky, saying, "I see him trying to back out of the rare earths agreement. If he does that, he's going to have some problems."

"Big, very big problems."

"We agreed on a deal on rare earths, and now he's saying, 'Well, you know, I want to renegotiate the deal.' He wants to be a member of NATO. Well, he was never going to join NATO. He understands that. So if he's looking to renegotiate the deal, he's going to have big problems."

The improvement in relations between Washington and Moscow since Trump took office, and his threat to halt support for Kyiv, has strengthened Russia's position on the battlefield as it continues its faltering invasion of Ukraine.

Kyiv accuses Moscow of prolonging the talks without any intention of stopping the war, amid a new attack launched over the weekend on the northeastern border city of Kharkiv.

Ukrainian regions were targeted by six airstrikes overnight Saturday to Sunday, wounding one person while receiving treatment in a military hospital and killing at least two people in a residential building, according to Ukrainian officials.

Moscow also announced on Sunday that its forces had taken control of a village just seven kilometers from the border with the Dnipropetrovsk region in central Ukraine, as part of its latest advance.

Russian forces have not crossed the region's borders since the start of their offensive in 2022, but have been advancing toward it for months, hoping to achieve a breakthrough.

Putin, who has been in power for 25 years and has been elected several times without any real competition, has frequently questioned Zelensky's "legitimacy" as president after the Ukrainian president's five-year term ends in May 2024.

Under Ukrainian law, elections are suspended during major military conflicts, while all of Zelensky's domestic opponents have declared that no elections should be held before the war ends.

Trump himself had a strained relationship with Zelensky, calling him a "dictator" and attacking him in front of reporters and the media during his visit to the White House last month.

In his Saturday evening address, Zelensky said, "For too long, America's proposal for an unconditional ceasefire has remained on the table without an appropriate response from Russia."

"There could have been a ceasefire if there was real pressure on Russia," he added, thanking countries that "understood this" and intensified sanctions pressure on the Kremlin.

Moscow and Kyiv agreed on a framework for a Black Sea truce after talks with US officials earlier this week, but Russia said the agreement would not take effect until Ukraine's allies lifted some sanctions.

Explaining his threat to impose secondary tariffs on Russian oil, Trump told NBC, "This means that if you buy oil from Russia, you can't do business in the United States."

He added, "There will be a 25% duty on all oil, and a customs duty of between 25 and 50 points on all oil," without elaborating.