Russia has sent tanks, heavy weapons and rocket launchers to Ukraine in recent days in support of separatists in the east of the country, the U.S. State Department said on Friday.
The confirmation by the United States of reports that Russian tanks had crossed the border into Ukraine is likely to deepen strains with Moscow.
"We assess that separatists in eastern Ukraine have acquired heavy weapons and military equipment from Russia, including Russian tanks and multiple rocket launchers," State Department spokeswoman Marie Harf said in a statement.
Harf told a briefing earlier that a convoy of three T-64 tanks, several MB-21 "or Grad" multiple rocket launchers and other military vehicles had crossed from Russia into Ukraine in the last three days.
"This is unacceptable," Harf told the briefing. "A failure by Russia to deescalate the situation will lead to additional costs."
The United States and European allies have threatened broader sanctions against Russia unless Moscow stops assisting rebels in eastern Ukraine.
In a statement, Harf said Ukrainian Interior Minister Arsen Avakov reported on Thursday that three tanks had crossed into Ukraine from Russia.
Internet video showed tanks, which were spotted at a deployment site in southwest Russia, moving through multiple cities in eastern Ukraine, she said.
"Russia will claim these tanks were taken from Ukrainian forces, but no Ukrainian tank unit has been operating in that area," Harf said. "We are confident that these tanks came from Russia,"
She said multiple rocket launchers, which were at the same deployment site in southwest Russia, were seen traveling through the Ukrainian city of Luhansk.
Ukraine's new president, Petro Porochenko, had spoken to Russia's Vladimir Putin on Thursday about the issue, while U.S. Secretary of State John Kerry raised concerns with his Russian counterpart Sergei Lavrov, Harf said.
Want to enjoy 'Zen' reading - with no ads and just the article? Subscribe todaySubscribe now