The Obama administration on Friday condemned the movement of a Russian convoy into eastern Ukraine, calling it a violation of Ukraine's sovereignty and demanding that it be withdrawn.
"We are deeply concerned about this," said Ben Rhodes, deputy national security adviser to President Barack Obama.
Rhodes said Russia would face additional costs if the convoy is not removed. He said the U.S. would discuss the matter Friday with its partners on the U.N. Security Council.
"Russia must remove its vehicles and its personnel from the territory of Ukraine immediately," the Pentagon's press secretary, Navy Rear Adm. John Kirby told reporters earlier. "Failure to do so will result in additional costs and isolation."
He said the U.S. is not sure of the contents of the Russian trucks. Reports from Ukraine said trucks carrying food, water, generators and sleeping bags crossed into rebel-held regions of Ukraine on Friday morning without the permission of the government in Kiev.
Ukraine called the Russian move a "direct invasion."
Kirby noted that Russia's defense minister had recently "guaranteed" Defense Secretary Chuck Hagel that the Russian military would not invade Ukraine.
At NATO headquarters in Brussels, the alliance's secretary general condemned Russia for sending a "so-called humanitarian convoy" into eastern Ukraine.
Anders Fogh Rasmussen called Russia's unilateral decision to send more than 130 trucks filled with what it called humanitarian aid into rebel-held areas "a blatant breach of Russia's international commitments" and "a further violation of Ukraine's sovereignty by Russia."