X has been hit by a "massive cyber attack", according to Elon Musk.
The boss of the social media platform said the attack was carried out "with a lot of resources".
Mr Musk claimed that either a large coordinated group or a country was involved, or both.
He wrote on the platform: "There was (still is) a massive cyberattack against X.
"We get attacked every day, but this was done with a lot of resources. Either a large, coordinated group and/or a country is involved. Tracing..."
Later on Monday, Mr Musk claimed on Fox News that IP addresses involved in the cyber attack were traced to locations "in the Ukraine area".
The US government has recently revoked some access to satellite imagery for Ukraine and paused intelligence sharing, piling pressure on Kyiv as President Trump seeks a swift end to the war.
In a now-deleted post on Telegram, a hacking group called Dark Storm Team claimed responsibility for the attack.
Around 40,000 users reported they could not access the platform on Monday, according to data from tracking website Downdetector.com.
Complaints about outages spiked at around 10am in the UK, and again at 2pm.
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A source in the internet infrastructure industry told the Reuters news agency that the platform had been hit by several waves of denial of service (DoS) attacks at around 9.45am.
According to the National Cyber Security Centre, DoS attacks are attempts to overload a website or network - aiming to degrade its performance or make it completely inaccessible.
Such attacks are not necessarily sophisticated but they can cause significant disruption.
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