Ukraine war: Zelenskyy warns of Russian plan to intensify invasion on second front; attack kills 16 at Kharkiv DIY shop

Russian forces are making "maximum efforts" to break through Ukraine's defences, amid fighting on the frontline near Kharkiv.

Kyiv's armed forces said there have been 83 clashes with the enemy on Sunday.

"Ukrainian troops are taking measures to maintain positions and destroy the offensive potential of the enemy," the General Staff said in an update on social media.

It said there were "no major changes" in the direction of Kharkiv but Russian troops continue to mount offensives around Vovchansk city and the village of Lyptsi.

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By John Sparks, international correspondent

If the war in Ukraine was fought in the media alone, the Russians would have been repelled months, or even years, ago.

While the Kremlin funds "debate shows" with far-right nationalists spouting historical theories from the 19th century, the Ukrainians fight their corner with slickly-produced content highlighting every aspect of their defence.

Want to see the Ukrainians blowing up Russian tanks? That will be on X.

The petition to send clapped out cars in London to the Ukrainian military? Head to "Insta".

The passionate address where President Volodymyr Zelenskyy speaks from a burnt-out book-making factory, pleading with Joe Biden and Xi Jinping to turn up at a peace summit?

That dropped this morning.

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German Chancellor Olaf Scholz has repeated his view that weapons donated to Ukraine should not be used to strike targets inside Russia.

There are "clear rules for German arms deliveries that have been agreed with Ukraine and that work," he said in Berlin earlier, according to Tagesschau television.

"That is my theory anyway," he said, explaining that the aim of his Ukraine policy is to "prevent this from becoming a really big war".

German arms deliveries to Ukraine come with the condition that the weapons not be used on Russian territory - similar to donations by many other NATO countries.

But - as we have discussed on this blog today - there is increasing discussion in the West about whether this is a sustainable policy.

Anton Hofreiter, from the rival Green Party, called at the weekend for Ukraine to be permitted to hit Russian territory with Western weapons. 

A 12-year-old girl is among those killed in the strike on the Ukrainian city of Kharkiv.

In an update this afternoon, Ukraine's ambassador to the US said the rescue operation is still ongoing.

Oksana Markarova said 16 people were killed including the girl and that 44 people were wounded.

She said on X: "Just yesterday I saw texts from 20 year old young woman who was looking for her mom and 12-year-old sister as their phones went silent. 

"Do not be silent. Help us stop Russia!" 

Volodymyr Zelenskyy has directly called on his US and Chinese counterparts to join his latest summit for peace in Ukraine.

"To president Biden, the leader of the United States, and to president Xi, the leader of China, we do not want the UN charter to be burned.

"Please show your leadership in advancing the peace."

A 13-year-old boy was pictured in the aftermath of the deadly strike on Kharkiv that killed at least 16 people.

Ukrainian MP Inna Sovsun shared the image, saying the boy was having to give a DNA sample in order to identify the body of his father, who died in the strike.

"Sometimes I think I cannot hate more. And then I do," Ms Sovsun said on X alongside the picture.

By Deborah Haynes, security and defence editor

Russia is producing artillery shells around three times faster than Ukraine's Western allies and for about a quarter of the cost, according to an analysis shared with Sky News.

The war has been described from the start as a "battle of fires" because of the volume of artillery rounds used.

Ukraine's allies have sought to ramp up production, but their ability still lags behind Moscow's.

As a result, Ukrainian soldiers on the frontline say for every one round they fire against Russian positions, the invading troops can launch around five shells back.

Battling against the odds, the Ukrainians say they have become skilled at trying to make every round count.

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In the aftermath of the deadly strike on the city of Kharkiv, Volodymyr Zelenskyy has renewed his call for Western countries to allow Ukraine to use donated weapons against targets in Russia.

The US and other NATO countries have thus far resisted Ukrainian requests to use their donated weapons outside of Ukraine's borders.

They fear escalation with Moscow, but the consequences of the policy - Ukraine says - were clear when Russian troops were able to mass across the border from Kharkiv.

Speaking on X this afternoon, President Zelenskyy said: "The world has the means to counter Russian terror, and we in Ukraine need this assistance. 

"Timely and sufficient support from partners. Steady and unwavering support for our warriors. 

"Additional modern air defence systems, particularly Patriots, and the ability to destroy Russian terrorists so that they cannot approach our borders." 

The number of Russian casualties since the beginning of the full-scale invasion has passed half a million, Ukraine says.

More than 1,000 more were either killed or wounded in the last 24 hours.

While other Western estimates on Russian losses differ from Ukraine's, it is nonetheless a sign of the staggering human cost of the war for Moscow.

Russia does not routinely publish numbers of casualties or losses of equipment. Ukraine provides some figures but it does so infrequently.

In February, Volodymyr Zelenskyy said that 31,000 of his troops had been killed in two years of war.