"We are concerned that Russia could try to use a humanitarian or peacekeeping operation as a pretense for inserting elements of its military force into Ukraine," the unnamed official told The Hill on Tuesday.
Nearly 300 Russian trucks have set off from Moscow, carrying 2,000 tons of aid, including medicine, food, sleeping bags and power generators. Ukraine, however, rejected the convoy.
Ukrainian Deputy Foreign Minister Danylo Lubkivsky said that there would be no need for humanitarian aid.
Pentagon spokesman Army Col. Steve Warren said on Tuesday, "If this is some sort of Trojan Horse, it would be illegal."
During a press briefing on Tuesday, State Department deputy spokeswoman Marie Harf condemned Moscow's aid move.
"Russia has no right to move into Ukraine unilaterally, whether under the guise of humanitarian convoys or any other pretext, without Kiev's permission," she said.
Comment: Notice how the U.S. catapults its propaganda with zero evidence, using only suggestion and conjecture: "could try", "if", "would be", "guise of". Harf's inner psychopath comes out here. Only a monster would condemn humanitarian aid to the people of east Ukraine.
When asked whether the convoy was an invasion, she replied, "Well, we don't know."
"We do have concerns. And that's why, as we've said today, if it goes through all of these steps, then we would support this," Harf said.
Meanwhile, Russian Foreign Minister Sergey Lavrov said Moscow and Kiev had agreed on a humanitarian mission under the authority of the Red Cross.
"I am certain there will be no disruptions on their part. They are on the territory whose residents need humanitarian aid," Lavrov said.
The Red Cross says that humanitarian situation in eastern Ukraine is critical as reports say thousands have no access to water, electricity and medical aid.