Claiming it is "ensuring security," Russia also moved troops to within a "handful of kilometers" from Ukraine territory, the U.S. official said.
And even as Ukrainian President Petro Poroshenko declared a weeklong ceasefire with the pro-Russian separatists, they do not appear to have any intention of laying down their arms and going along with the ceasefire, according to reports from the ground.
The Ukrainian government briefed Western diplomats in Kiev on Friday and told them it has evidence that 10 additional tanks, along with fuel trucks and supporting vehicles, crossed the border between the countries in the last 24 hours.
The U.S. official said the U.S. government has independently confirmed additional tanks departed from a deployment site in southwest Russia on Thursday.
Amid the new developments, the U.S. sanctioned seven pro-Russian separatists, including their self-professed "commander," Igor Strelkov, for their actions in destabilizing the sovereignty and territorial integrity of Ukraine.
The U.S. official again warned Russia continued steps to escalate the situation and a failure to accept a peace plan proposed by new Poroshenko would result in further "costs."
"If its destabilization of Ukraine does not abate and it does not support this peace plan, there will be more costs. More costs in the form of isolation and sanctions," the official said.
The official said the U.S. has picked up talks with the EU in recent days about potential "scalpel" sanctions against Russia.
In early May, President Obama and German Chancellor Angela Merkel explicitly threatened Russian President Vladimir Putin with the possibility of crippling sectoral sanctions before the crisis in Ukraine seemed to calm down.
"The idea here is to deny Russia the kinds of investment and next-generation technology that it needs to continue to grow," the official said of the possible sanctions.
A week ago, the U.S. confirmed Russia had sent convoy of three T-64 tanks, several BM-21, or Grad multiple rocket launchers, and other military vehicles into Ukraine.
A deputy spokeswoman at the State Department called the escalation "unacceptable."
Source: Business Insider