Zelensky made the remarks at a security forum in Singapore as he appealed for more military aid for Ukraine, which has been ceding ground to Russian troops.

Ukraine is also trying to whip up attendance at the peace meeting in Switzerland this month where it hopes to win broad international backing for its vision of the terms needed to end Russia's war.

Zelensky said more than 100 countries and organisations had signed up to the conference, and he urged Asia-Pacific nations to join.

"We are disappointed that some world leaders have not yet confirmed their participation in the peace summit," Zelensky said at the Shangri-La Dialogue, without mentioning China or the United States by name.

The peace summit threatens to be overshadowed if key Ukraine backer Biden -- who is locked in campaigning against Donald Trump and has given no sign he would participate -- ultimately stays away.

China also said Friday it would be "difficult" to participate in the conference if Russia did not participate.

Beijing believes the conference "should have the recognition of Russia and Ukraine, equal participation of all parties and fair discussion of all peace plans", foreign ministry spokeswoman Mao Ning told a regular news conference.

"Otherwise, it is difficult for the conference to play a substantive role in restoring peace," she said.

- 'Unwavering US support' -

Zelensky also met with US Defence Secretary Lloyd Austin on Sunday on the sidelines of the Singapore security forum. Zelensky told reporters they had a "very good" meeting.

In a post on X, Zelensky said the pair discussed "the defense needs of our country, bolstering Ukraine's air defense system, the F-16 coalition, and drafting of a bilateral security agreement."

In a readout of the talks, Pentagon spokesman Major General Pat Ryder said Austin reiterated "unwavering US support for Ukraine in the face of Russian aggression".

Austin also "reaffirmed the US commitment to maintaining the strong support of a coalition of over fifty countries to help Ukraine defend its freedom," Ryder said.

The meeting came after Washington decided to partially lift restrictions on using US-provided weapons to strike inside Russia, which Zelensky has hailed as a "step forward".

Zelensky and Austin last met face-to-face in December in Washington, where Zelensky made a last-ditch plea for US aid before it ran out.

The US Congress approved in April a $61-billion aid package for Ukraine following months of political wrangling, unlocking much-needed arms for the country's outgunned troops.

Chinese defence spokesman Wu Qian told AFP he was not aware of any plans for Chinese Defence Minister Dong Jun to meet with Zelensky in Singapore.

Ukraine is struggling to hold back a Russian ground offensive in the Kharkiv region, where Moscow recently made its largest territorial gains in 18 months.

Zelensky has been touring European nations in recent days seeking more military aid for Ukrainian troops, and warning partners of the risks if they show any signs of fatigue over the war.

"It is very important for Ukrainians that the world does not get tired... that the world understands that it cannot get tired of the war launched by the aggressor," Zelensky said in Portugal on Tuesday.