Two reporters of Russian website Life News were reportedly arrested by Ukrainian troops near the city of Kramatorsk, prompting a furious reaction from Moscow and demands they be immediately 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, according to reports, detained a number of individuals who were in possession of fake journalist credentials issued by the nonexistent Donetsk People's Republic."
"And certainly that raised some questions about these individuals and whether they were actually journalists."
Quizzed by reporters in Washington for more information about the weapons, Psaki added: "These are according to reports and our conversations with the Ukrainians on the ground."
Separately, several journalists complained Monday of arrests by Russian security services in Crimea, the peninsula annexed by Moscow.
"We would condemn, of course, the unlawful detention of journalists in any capacity," said Psaki.
