Ukraine is ready to hold direct peace talks with Russia but only after a ceasefire is in place, President Volodymyr Zelensky said Tuesday amid new US pressure to end the three-year-old conflict.

US presidential envoy Steve Witkoff is to go to Moscow this week, the White House said, and a US envoy was to take part in new talks with European officials in London on Wednesday.

US media reports have said US President Donald Trump has proposed accepting that Moscow-annexed territory in Crimea be recognised as Russian and this will be discussed.

"After the ceasefire, we are ready to sit down in any format," Zelensky told journalists at a briefing a day before the key talks in London on a potential Ukraine settlement.

Trump, who had promised to strike a Moscow-Kyiv deal within 24 hours of taking office, has failed in the three months since to secure concessions from Russian President Vladimir Putin.

Trump said at the weekend that he hoped a peace deal could be struck "this week" despite no signs the two sides were close even to a ceasefire, let alone a long-term settlement.

No 'rigid time frames'

Kremlin spokesman Dmitry Peskov said Tuesday that the conflict was too "complex" to achieve a speedy ceasefire.

"It is not worth setting any rigid time frames and trying to get a settlement, a viable settlement, in a short time frame," he told state TV.

White House spokeswoman Karoline Leavitt confirmed Witkoff would be travelling to Moscow, his fourth visit to Russia since Trump took office.

"The negotiations continue. We're hopefully moving in the right direction," she said.

Trump "has grown frustrated with both sides of this war, and he's made that very known."

Moscow's forces occupy around a fifth of Ukrainian territory and tens of thousands of people have been killed since the war started in February 2022.

After rejecting a US-Ukrainian offer for an unconditional ceasefire last month, Putin announced a surprise Easter weekend truce. Fighting dipped during the 30-hour period but Russia launched fresh attacks on residential areas on Monday and Tuesday, Ukrainian officials said.

Ukraine's foreign ministry meanwhile called in China's ambassador to raise "serious concern" over allegations that Chinese fighters were in the Russian army and Chinese companies were helping Russia make military hardware, the ministry said.

Zelensky has said at least 155 Chinese were fighting with the Russian army — two of whom have been detained by Ukraine — and that he had "information" that China was supplying arms to Russia. China last week denied providing weapons.

The foreign ministry summoned China's envoy in Kyiv, Ma Shengkun, over the accusations and produced "evidence" to back the claims, the ministry said.

London meeting

Ukraine's allies are expected to discuss a possible deal they could all get behind when they meeting in London on Wednesday, a senior Kyiv official told AFP.

US Secretary of State Marco Rubio will not attend the London talks due to scheduling issues, the State Department said, adding that US envoy for Ukraine Keith Kellogg would take part.

European leaders are scrambling to work out how to support Ukraine should Trump pull Washington's vital military and financial backing.

Zelensky said his team's "first priority" at the London talks would be "an unconditional ceasefire."

On Sunday he proposed a halt of missile and drones strikes against civilian facilities for at least 30 days.

While saying he would "analyze" the idea, Putin threw doubt on it later by accusing Kyiv of using civilian facilities for military purposes.

He held open the prospect of bilateral talks, though Peskov said there were "no concrete" plans to engage with Kyiv. "There is readiness from Putin to discuss this question," he  said.

"If we are talking about civilian infrastructure, then we need to understand, when is it civilian infrastructure and when is it a military target," he added.

Russian attacks

Russia hit a residential area in the eastern Ukrainian city of Myrnograd with drones Tuesday, killing three people and wounding two, authorities said.

One person was reported dead and 23 wounded after two aerial bombs pounded the southern city of Zaporizhzhia, the region's governor said.

Photos from Ukraine's emergency services showed the outer walls of an apartment block blown open and a bloodied man on a stretcher, with bandages around his head and arms.

Russian strikes wounded another six people in the southern city of Kherson and seven in Kharkiv in the northeast, officials said.

The Russian army meanwhile claimed to have captured a village in the eastern Donetsk region, where its troops are advancing.

Russia has advanced in recent months in southern and eastern Ukraine and recaptured much of Russia's Kursk region, parts of which Kyiv seized last year and was hoping to use as a bargaining chip.

There were no ongoing discussions on any new US aid packages with the Trump administration, Zelensky said.

In Paris last week, Rubio presented Washington's plan for ending the conflict, though both he and Trump warned that Washington's patience was wearing thin and could lead it to withdraw.