President Volodymyr Zelensky has pledged that Ukraine is ready to work quickly to produce a strong agreement on investments and security with the United States, saying a meeting with US envoy Keith Kellogg "restores hope" for success.

"General Kellogg, a meeting which restores hope. We need strong agreements that will really work. I gave instructions to work fast and in a very, very even-handed fashion," Zelensky said in his nightly video address.

"The details of the agreement are important. The better the details are drafted, the better the result."

Zelensky delivers his nightly video address. – Reuters

The meeting with Kellogg took place a day after Zelensky and US President Donald Trump exchanged barbs as US-Russian talks got under way on ending the three-year-old war pitting Kyiv against Moscow. Ukraine was not invited to the talks.

After the meeting with Kellogg, Zelensky said on social media platform X that Ukraine had to "ensure that peace is strong and lasting – so that Russia can never return with war."

"Ukraine is ready for a strong, effective investment and security agreement with the President of the United States.

"We have proposed the fastest and most constructive way to achieve results. Our team is ready to work 24/7."

People in Kharkiv react as Trump warns Zelensky to quickly negotiate war's end. – AP

The talks with Kellogg also followed Ukraine's rejection of an initial US proposal to develop rare earths in Ukraine.

In his comments on X, Zelensky also said his discussion with Kellogg focused on the battlefield situation, the security guarantees that Ukraine is seeking and the return of prisoners of war.

"It's important for us – and for the entire free world – that American strength is felt," he wrote.

Trump is seeking to reestablish ties with Russia and also invest in Ukraine's mineral resources critical to the energy transition.

Ukraine rejected an initial US plan as it did not include security guarantees.

The Trump administration is reportedly considering presenting a simplified minerals deal to Zelensky, though the Ukrainian president made no mention of the revised agreement.

Scholz voices support for Zelensky as 'democratically elected president'. – Reuters

European leaders have responded to Trump's stance on Ukraine by pledging to step up spending on defence and some are considering a US-backed European peacekeeping force for the country.

French President Emmanuel Macron, who is due to meet with Trump in Washington on Monday, said he would tell Trump not to be "weak" on Russian President Vladimir Putin.

"How can you be credible with China if you're weak with Putin?" Macron said, during a question-and-answer session on social media.

Waltz, for his part, said the US would welcome European-backed security guarantees for Ukraine, and he pushed for all NATO members to be spending at least 2 per cent of their GDP on defence by the alliance's next summit, set for June.

The Kremlin says the European plan is a major cause for concern but Zelensky and NATO have welcomed it.

"It is vital that … Russia will never again try to take one more square kilometre of Ukrainian land," NATO Secretary-General Mark Rutte said.

Robust security guarantees needed for lasting peace in Ukraine, NATO chief says. – Reuters

Russian forces have laid Ukrainian cities, towns and villages to waste and are edging forward along parts of the 1000km (600-mile) frontline across eastern and southern Ukraine. Moscow controls a fifth of Ukraine and claims ownership of more.

Ukrainian officials say a ceasefire would just give Russia time to prepare for further aggression. However, the head of Ukrainian military intelligence agency said there could be a ceasefire this year, while casting doubt on its durability.

At the centre of Trump's charge that Zelensky is a dictator is that Ukraine has not held elections because of martial law, which it declared when Russia unleashed its invasion on February 24, 2022.

Martial law, which provides authorities with emergency powers for the war effort, prohibits holding elections. Zelensky won office in 2019 and his mandate would normally have ended last May.

Trump's comments spurred some, though not all, of Ukraine's opposition figures to rally around Zelensky.

Ukrainians voice support for Zelensky after Trump calls him a 'dictator'. – Reuters

Ex-prime minister Yulia Tymoshenko said Zelensky was Ukraine's legitimate leader until someone else was elected, and that it was "impossible and immoral" to hold elections during the war, as the military would not be able to take part.

"Only Ukrainians have the right to decide when and under what conditions they should change their government. Today, there are no such conditions!" she wrote on Facebook.

Valeriy Zaluzhnyi, the ex-army chief who is Ukraine's ambassador to London and whose popularity makes him a possible presidential candidate, said winning the war with Russia to preserve Ukraine, not elections, was the priority.

The most prominent opposition figure who has not pushed back on Trump's latest election call is Petro Poroshenko, the former president whose relations with Zelensky are acrimonious.

Poroshenko previously opposed calls for wartime elections in the name of national unity but remained silent this time after the government imposed sanctions on him last week, something he said was a blow to unity.

Kremlin says Ukraine’s statements about heads of other states 'unacceptable'. – AP

Iryna Herashchenko, a lawmaker for Poroshenko's party, has been calling for a government of national unity and urged Zelensky to stop what she called "political repression against those he does not like".

Serhii Prytula, who runs a major charity supporting the Ukrainian army and is another possible presidential hopeful, advised Ukrainians on X not to read Trump's comments before bed.

"Remember that here in Ukraine, only we – the people of Ukraine – determine who is a dictator and who is not."