Meta, TikTok, Twitter and YouTube have received notice from American politicians asking them to preserve content that could be used to prosecute war criminals. 

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