Ukrainian President Volodymyr Zelenskyy met on Friday with President Donald Trump at a pivotal moment for his country, as he tries to persuade the White House to provide some form of US backing for Ukraine's security against any future Russian aggression.

Zelenskyy flashed a thumb's up but didn't speak to reporters as he arrived at the White House. He wore a more formal black top rather than his usual military green T-shirt, prompting Trump to quip, “He's all dressed up.”

Zelenskyy's delegation is expected to sign a landmark economic agreement with the US aimed at financing the reconstruction of war-damaged Ukraine, a deal that would closely tie the two countries together for years to come.

The deal, which is seen as a step toward ending the three-year war, references the importance of Ukraine's security. But it leaves that to a separate agreement still being hammered out by the two leaders.

Meeting together in the Oval Office, Trump said the agreement would be signed soon in the East Room of the White House.

“We have something that is a very fair deal,” Trump said, adding, “It is a big commitment from the United States.”

He said the US wants to see the killing in the war stopped, adding that US money for Ukraine should be "put to different kinds of use like rebuilding”.

Zelenskyy called Russian President Vladimir Putin a terrorist and told Trump that Ukraine and the world need “no compromises with a killer”.

“Even during the war there are rules,” he said.