U.S. President Donald Trump said early on Friday he came away disappointed from a telephone call with Russian President Vladimir Putin because it does not appear the latter is looking to stop Russia's war against Ukraine.

U.S. attempts to end Russia's war in Ukraine through diplomacy have largely stalled, and Trump has faced growing calls - including from some Republicans - to increase pressure on Putin to negotiate in earnest.

After speaking to Putin on Thursday, Trump plans to speak to Ukraine President Volodymyr Zelenskyy on Friday, he said in remarks to reporters on his return to Washington from a trip to Iowa.

"I'm very disappointed with the conversation I had today with President Putin, because I don't think he's there, and I'm very disappointed," Trump said.

"I'm just saying I don't think he's looking to stop, and that's too bad."

The two leaders did not discuss a recent pause in some U.S. weapons shipments to Kyiv during the nearly hour-long conversation, a summary provided by Putin aide Yuri Ushakov showed.

Within hours of their concluding the call, an apparent Russian drone attack sparked a fire in an apartment building in a northern suburb of Kyiv, Ukrainian officials said, indicating little change in the trajectory of the conflict.

In Kyiv itself, Reuters witnesses reported explosions and sustained heavy machine-gun fire as air defence units battled drones over the capital, while Russian shelling killed five people in the east.

"I didn't make any progress with him at all," Trump told reporters on Thursday.

Zelenskyy told reporters in Denmark earlier in the day that he hopes to speak to Trump as soon as Friday about the pause in some weapons shipments first disclosed this week.

Speaking to reporters as he left Washington for Iowa, Trump said "we haven't" completely paused the flow of weapons but blamed his predecessor, Joe Biden, for sending so many weapons that it risked weakening U.S. defenses.

"We're giving weapons, but we've given so many weapons. But we are giving weapons," he said.

"And we're working with them and trying to help them, but we haven't (completely stopped). You know, Biden emptied out our whole country giving them weapons, and we have to make sure that we have enough for ourselves."

The diplomatic back-and-forth comes as low stockpiles have prompted the U.S. to paused shipments of certain critical weapons to Ukraine, sources told Reuters earlier, just as it faces a Russian summer offensive and growing attacks on civilian targets.

Putin has continued to assert he will stop his invasion only if the conflict's "root causes" have been tackled, making use of Russian shorthand for the issue of NATO enlargement and Western support for Ukraine, including the rejection of any notion of Ukraine joining the NATO alliance.

Russian leaders are also angling to establish greater control over political decisions made in Kyiv and other Eastern European capitals, NATO leaders have said.

The pause in U.S. weapons shipments caught Ukraine off-guard and has generated widespread confusion about Trump's current views on the conflict, after saying just last week he would try to free up a Patriot missile defense system for use by Kyiv.

Ukrainian leaders called in the acting U.S. envoy to Kyiv on Wednesday to underline the importance of military aid from Washington, and caution that the pause in its weapons shipments would weaken Ukraine's ability to defend itself against Russia.

The Pentagon's move has meant a cut in deliveries of the Patriot defense missiles that Ukraine relies on to destroy fast-moving ballistic missiles, Reuters reported on Wednesday.

Kremlin Aide Yuri Ushakov, said that while Russia was open to continuing to speak with the U.S., any peace negotiations needed to happen between Moscow and Kyiv.

That comment comes amid some signs that Moscow is trying to avoid a three-way format for possible peace talks.

The Russians asked American diplomats to leave the room during such a meeting in Istanbul in early June, Ukrainian officials have said.

Ushakov also added that Trump and Putin did not talk about a face-to-face meeting .

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