By Matt Murphy
Ukraine has accused Russia of attacking the besieged city of Bakhmut with phosphorus munitions.
In drone footage released by Ukraine's military, Bakhmut can be seen ablaze as what appears to be white phosphorus rains down on the city.
White phosphorus weapons are not banned, but their use in civilian areas is considered a war crime.
They create fast-spreading fires that are very difficult to put out. Russia has been accused of using them before.
Russia has been trying to capture Bakhmut for months, despite its questionable strategic value. Western officials have estimated that thousands of Moscow's troops have died in the assault.
Writing on Twitter, Ukraine's defence ministry said the attack had targeted "unoccupied areas of Bakhmut with incendiary ammunition".
It is unclear when it took place. But the footage shared by Ukraine - seemingly captured by a surveillance drone - showed high-rise buildings engulfed in flames.
Other videos posted to social media showed fires raging on the ground and white clouds illuminating the night sky.
A BBC analysis of the video posted by the defence ministry located the footage to an area just west of Bakhmut city centre and close to a children's hospital. While the analysis confirmed the attack used some kind of incendiary munitions, it could not verify the use of phosphorus.
Russia has been accused of using white phosphorus in Ukraine, including during the siege of Mariupol at the beginning of the war.
Moscow has never publicly admitted to using the substance, and last year Kremlin press secretary Dmitry Peskov insisted "Russia has never violated international conventions" after Ukrainian President Volodymyr Zelensky said it had been used.
White phosphorus is a wax-like substance that burns at 800 degrees Celsius and ignites on contact with oxygen, creating bright plumes of smoke.
Human Rights Watch (HRW) has warned the chemical is "notorious for the severity of the injuries it causes".
It is extremely sticky and hard to remove, and can re-ignite when bandages are removed.
Russia has signed the Convention on Certain Conventional Weapons, which bans the use of incendiary weapons - which are designed to catch fire - in civilian areas.
But HRW said white phosphorus does not fall under the treaty as its primary purpose is to "create a smokescreen to hide military operations".
The chemical has been used "repeatedly over the past 15 years", including by US forces against IS fighters in Iraq and Syria, according to HRW.
Some analysts say its use as an incendiary weapon near civilians would still be illegal. While Bakhmut had a pre-war population of 80,000, there are practically no civilians left in the area.
The attack comes a day after the commander of Russia's Wagner paramilitary group said he would pull his forces out of Bakhmut on 10 May in a row over ammunition supplies.
Yevgeny Prigozhin said Wagner's casualties were "growing in geometrical progression every day", and blamed the defence ministry for his decision to withdraw.
On Saturday, Prigozhin said Ramzan Kadyrov - the leader of Russia's semi-autonomous Chechnya region - had agreed to take over Wagner positions in the city and replace their fighters with his own.
"I am already contacting his representatives in order to start transferring positions immediately, so that at 00:00 on 10 May," Prigozhin's press service quoted him as saying.
Despite his claims, Ukrainian officials said Wagner was redeploying mercenaries towards Bakhmut in a bid to capture the city before Tuesday's Victory Day celebrations in Russia.
"We are now seeing them pulling [fighters] from the entire offensive line where the Wagner fighters were, they are pulling [them] to the Bakhmut direction," Deputy Defence Minister Hanna Maliar said.
The fighting comes amid reports that Ukraine is preparing a large-scale counteroffensive. Prigozhin himself has said he believes the attack could come as soon as 15 May.
An offensive could take place in the Zaporizhzhia region which is about 80 percent controlled by Russia.
On Friday, the Russian-installed governor of Zaporizhzhia ordered the evacuation of villages near the front line.
Russia considers the area as its own territory, following self-styled referendums and an illegal annexation last year.
Russian pro-war blogger injured in car bomb
A prominent Russian writer and pro-war blogger has been wounded by a car bomb, officials say.
Zakhar Prilepin, an vehement supporter of the Ukraine war, was reportedly conscious after the attack in the Nizhny Novgorod region but his driver was killed.
A suspect was detained, the interior ministry said.
It comes a month after another pro-Kremlin blogger, Vladlen Tatarsky, died in a bombing at a St Petersburg café.
Less that two hours after Saturday's attack was reported, the interior ministry said a suspect with previous convictions had been detained near a forest in the Nizhny Novgorod region.
"The search for possible accomplices continues," the statement said.
Nizhny Novgorod Governor Gleb Nikitin said: "Law enforcement officers are now investigating the circumstances and causes of the incident. Zakhar is OK."
The explosion reportedly took place on a remote road some 80 kilometres from the town of Bor. State media said the blogger suffered a concussion and fractured bones.
The partisan group Atesh, which is made up of Ukrainians and Crimean Tartars, claimed it was behind the attack.
"We had a feeling that sooner or later he would be blown up," they wrote on Telegram. "He was not driving alone, but with a surprise on the underside of the car."
The BBC cannot verify Atesh's claims.
As well as being one one of Russia's best-known novelists, Prilepin is known for his involvement with Russian ultranationalist politics.
A veteran of Russia's bloody wars in Chechnya in the 1990s, the 47-year-old has admitted fighting alongside pro-Russian separatists in eastern Ukraine.
He has called for the "return of Kyiv to Russia". Last year a group founded by Prilepin called on officials to "purge the cultural space" of all who oppose the conflict.
Kremlin spokesperson Dmitry Peskov declined to comment on the alleged bombing until the investigation was complete.
But Russian foreign ministry spokeswoman Maria Zakharova sought to blame the attack on the UK and the US.
"The fact has come true: Washington and Nato fed another international terrorist cell - the Kiev regime," she wrote on Telegram. "We pray for Zakhar."
The attack is the latest to target high-profile supporters of President Putin's war in Ukraine.
Vladlen Tatarsky was killed last month. The blogger had reported from the Ukraine front line and gained notoriety last year after posting a video filmed inside the Kremlin in which he said: "We will defeat everyone, we will kill everyone, we will rob everyone as necessary. Just as we like it."
Activist Darya Trepova, 26, was later arrested and was charged with terrorism following the publication of a video - believed to have been recorded under duress - in which she admitted bringing a statuette to the café that later blew up.
And in August 2022, Darya Dugina - the daughter of a close ally of Russian President Valdimir Putin - was killed in a suspected car bombing near Moscow.
It is thought her father, the Russian philosopher Alexander Dugin, who is known as "Putin's brain", may have been the intended target of that attack.