Two Russians that Moscow has accused Kiev of holding captive in eastern Ukraine were carrying anti-aircraft missiles, the US said.
Two reporters from Russian website Life News were reportedly arrested by Ukrainian troops near Kramatorsk, prompting a furious reaction from Moscow and demands they be released.
The website said reporters Oleg Sidyakin and Marat Saichenko were in the custody of the Ukrainian national guard who were "using violence" against them.
But US State Department spokeswoman Jen Psaki said: "The Ukrainian security services detained a number of individuals who were in possession of fake journalist credentials issued by the non-existent Donetsk people's republic.
"Reportedly, they were carrying manned portable anti-aircraft missiles at the time of their detention … that raised some questions about … whether they were actually journalists."
Quizzed by reporters in Washington for more information about the weapons, Psaki said: "These are according to reports and our conversations with the Ukrainians on the ground."