The United States believes Russia used a short-range ballistic missile to strike a railway station in east Ukraine on Friday, a senior US defence official said.
Ukraine said at least 50 people were killed and many more wounded in a strike on a station in the city of Kramatorsk that was packed with civilians hoping to flee the threat of a major Russian offensive.
The US defence official, speaking on condition of anonymity, said the Pentagon believes Russian forces used an SS-21 Scarab missile in the strike but that the motivation for the attack was not clear.
The SS-21 is the name used by the NATO military alliance for a type of missile known as the Tochka in former Soviet states.
The United States was still analyzing the strike and it was unclear whether cluster munitions were used, the US official said.
"We are not buying the denial by the Russians that they weren't responsible," the official said.
The Russian defence ministry was quoted by the RIA news agency as saying the missiles said to have struck the station were used only by Ukraine's military and that Russia's armed forces had no targets assigned in Kramatorsk on Friday.
Videos posted on social media in recent weeks, which Reuters could not independently verify, appear to show Russian forces in or near Ukraine transporting Tochka missile launchers.
The US defence official said Russia's combat power in Ukraine continued to decline and was somewhere between 80% and 85% of its pre-invasion levels.
The United States has estimated Russia assembled more than 150,000 troops around Ukraine before its invasion on Feb. 24.
The official said the United States now had indications Moscow has started mobilizing some reservists and could be looking to recruit more than 60,000 personnel.
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