Washington DC - US President Donald Trump said Friday he was "very unhappy" about his telephone call with Vladimir Putin on the war in Ukraine, saying the Russian leader just wanted to "keep killing people."

"It's a very tough situation. I told you I was very unhappy with my call with President Putin. He wants to go all the way, just keep killing people, it's no good," Trump told reporters aboard Air Force One.

Trump also hinted that he may finally be ready to toughen sanctions against Russia, having held off for the past six months while he tried to persuade Putin to end the war.

"We talk about sanctions a lot," Trump said. "He understands that it may be coming."

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Trump added that he and Ukrainian President Volodymyr Zelensky meanwhile had a "very strategic call" on Friday, as concerns mounted in Kyiv over US military aid deliveries.

Zelensky said earlier that the two leaders had agreed to work to "strengthen" Ukraine's air defenses, following Russia's largest drone and missile barrage of the invasion so far.

Trump said he had also discussed sending Patriot interceptor missiles to Ukraine in a separate call with German Chancellor Friedrich Merz on Friday, although he had not yet agreed to do so.

Merz "feels they have to be protected," Trump said.