Diplomatic efforts to end the war in Ukraine were stepping up on Monday, with Ukrainian and Russian negotiators set to talk again after both sides cited progress, even after Russia attacked a base near the Polish border and fighting raged elsewhere.

A barrage of Russian missiles hit Ukraine's Yavoriv International Centre for Peacekeeping and Security, a base just 15 miles (25 km) from the Polish border that has previously hosted NATO military instructors, killing 35 people and wounding 134, a Ukrainian official said on Sunday.

At least 35 dead in air strike on Ukraine military base near Poland

Russia's defence ministry said up to 180 "foreign mercenaries" and a large number of foreign weapons were destroyed. Reuters could not independently verify the casualties reported by either side.

Thousands of people have died since Feb 24, when Russian President Vladimir Putin launched what he called a special military operation to rid Ukraine of dangerous nationalists and Nazis.

The United States, which had watched Russia's build-up on Ukraine's borders with mounting alarm for weeks, says it was a premeditated, unjustified and unlawful "war of choice".

In a telephone call, US President Joe Biden and France's Emmanuel Macron underscored their commitment to holding Russia accountable for the invasion, the White House said.

US Secretary of State Antony Blinken and his Ukrainian counterpart, Dmytro Kuleba, also discussed diplomatic efforts to stop Russia's invasion, the State Department said.

Hopes were boosted after Russia and Ukraine gave their most upbeat assessments after weekend negotiations.

"Russia is already beginning to talk constructively," Ukrainian negotiator Mykhailo Podolyak said in a video online. "I think that we will achieve some results literally in a matter of days."

A Russian delegate to the talks, Leonid Slutsky, was quoted by the RIA news agency as saying they had made significant progress and it was possible the delegations could soon reach draft agreements.

Neither side said what these would cover. Three rounds of talks between the two sides in Belarus, most recently last Monday, had focused mainly on humanitarian issues.

Ukrainian President Volodymyr Zelenskiy said the countries' delegations have been speaking daily by video link and a clear aim of his negotiators was to "do everything" to arrange for him to meet Putin.

"We must hold on. We must fight. And we will win," Zelenskiy said in a late night video speech.

Harsh sanctions

Global financial markets, battered by fears the conflict could spread and drag in NATO, rallied on hopes for progress in peace talks. Stocks rose while oil prices gave up some of their massive recent gains.

Western nations have sought to isolate Putin by imposing harsh economic sanctions and supplying Ukraine with weapons.

Increasingly isolated, Russia asked China for military equipment after its invasion, sparking concern in the White House that Beijing may undermine Western efforts to help Ukrainian forces defend their country, several US officials said.

US National Security Adviser Jake Sullivan, who is due to meet China's top diplomat Yang Jiechi in Rome on Monday, warned Beijing it would "absolutely" face consequences if it helped Moscow evade sanctions.

Asked about Russia's request for military aid, Liu Pengyu, spokesperson for China's embassy in Washington, said, "I've never heard of that."

He said China found the current situation in Ukraine "disconcerting" and added, "We support and encourage all efforts that are conducive to a peaceful settlement of the crisis."

Still, violence and bloodshed continued.

Air raid sirens sounded before dawn in many cities and regions of Ukraine, including Kyiv, Lviv, Odessa, Ivano-Frankivsk and Cherkasy.

In the capital, authorities said they were stockpiling two weeks' worth of food for the 2 million people who have not yet fled from Russian forces attempting to encircle the city.

An American journalist was shot and killed by Russian forces in the town of Irpin, northwest of Kyiv, and another journalist was wounded, the regional police chief said.

Britain's defence ministry said Russian naval forces had established a distant blockade of Ukraine's Black Sea coast, isolating the country from international maritime trade.

In eastern Ukraine, Russian troops were trying to surround Ukrainian forces as they advance from the port of Mariupol in the south and the second city Kharkiv in the north, it added.

The Russian-controlled eastern city of Donbass and Crimea - which was seized by Russia in 2014 - are now connected by a road controlled by pro-Russian forces, Russia's RIA news agency quoted Georgy Muradov, the deputy prime minister of the Russian-backed government in Crimea, as saying.

The report could not be independently confirmed.

Russia's invasion has sent more than 2.5 million people fleeing across Ukraine's borders and trapped hundreds of thousands in besieged cities.

"It is terrifying how violent and inhuman it is," Olga, a refugee from Kyiv, told Reuters after crossing into Romania.

The United Nations says at least 596 civilians have died since the invasion began and the toll is probably considerably higher as it is difficult to confirm deaths in places such as Mariupol.

The city council in Mariupol said 2,187 residents had been killed since the start of the invasion. Reuters was not able to verify that toll.

Moscow denies targeting civilians. It blames Ukraine for failed attempts to evacuate civilians from encircled cities, an accusation Ukraine and its Western allies strongly reject.

Kharkiv, in northeastern Ukraine, has suffered some of the heaviest bombardment and dozens of civilians have been killed.

"We will stitch up the wounds and the pain of our country and our city," said Aliev, a 23-year-old musician. "We're not going anywhere."