US politicians have written to the bosses of Meta, TikTok, Twitter and YouTube to request that the companies archive content that includes potential evidence of Russian war crimes in Ukraine.

The letters were sent by four senior Democrats who chair powerful committees in the House of Representatives relating to oversight, foreign affairs and national security.

Although the letters are not equivalent to binding legal orders they are backed by considerable political power and come as the platforms face a range of pressures regarding online safety issues.

One of the letters seen by NBC News, addressed to Mark Zuckerberg, states: "We write to encourage Meta to take steps to preserve and archive content shared on its platforms that could potentially be used as evidence."

It adds this would assist "the US government and international human rights and accountability monitors [who will] investigate Russian war crimes, crimes against humanity, and other atrocities in Ukraine."

Other key developments:

• Relatives and supporters of the Ukrainian fighters holed up in the Azovstal steel plant under bombardment in the southern port of Mariupol demonstrated in Kyiv, pleading for them to be rescued

• One of Russian President Putin's closest allies said the West's increasing military support to Ukraine risked war between Russia and NATO

• German industrial giant Siemens AG says it is exiting Russia, where it has operated for almost 170 years

• Pressure on Europe to secure alternative gas supplies increased after Moscow imposed sanctions on European subsidiaries of state-owned Gazprom and Ukraine stopped a gas transit route, pushing prices higher

More than 10,000 potential war crimes under investigation

A senior British diplomat has recorded evidence of "credible allegations" of sexual violence against children by Russian troops, alongside other violations of UN Security Council rules on young people in wartime.

Dame Barbara Woodward, Britain's ambassador to the UN, said there was evidence "Russia is committing four of the Security Council's six grave violations against children in times of war".

Counter-terror police in London gathering evidence of potential war crimes said they have been struck by the "incredibly harrowing" material and eyewitness accounts from the front line.

Ukraine's prosecutor general office has said it is looking into more than 10,700 potential war crimes involving more than 600 suspects, including Russian soldiers and government officials.

The trial of a Russian soldier accused of killing a civilian is currently underway in Kyiv. Sergeant Said Shyshimarin, 21, is accused of shooting a 62-year-old man in the head in the northeastern village of Chupakhivka.

It is the first time a member of the Russian military has been prosecuted for a war crime in the conflict.