"Today, in violation of its previous commitments and international law, Russian military vehicles painted to look like civilian trucks forced their way into Ukraine," Xinhua quoted National Security Council spokesperon Caitlin Hayden as saying in a statement.
"We condemn this action by Russia, for which it will bear additional consequences," she added, calling for an "immediate" withdrawal of Russian vehicles and personnel.
The Pentagon also urged Russia to recall its vehicles and personnel. "Failure to do so will result in additional costs and isolation," spokesperson John Kirby told reporters at a daily press briefing.
Russian media said about 130 trucks of the Russian convoy had entered Ukraine without escort of the International Committee of the Red Cross, with some having reached Lugansk, a rebel-held city that Ukrainian forces are trying to recapture.
Ukraine called the move "a direct invasion," while Russia said it was a result of intolerance of Kiev's deliberate delay.
"We warn against any attempts to undermine the completely humanitarian mission," the Russian foreign ministry said in a statement.
Further delay of Moscow's humanitarian mission would have been "unacceptable," Russian President Vladimir Putin told German Chancellor Angela Merkel over the phone on Friday.
Russia has been urging Ukraine to let in its convoy over the past week, while Kiev and the West were wary of Moscow's true intentions.
"Russia has no right to send vehicles, people, or cargo of any kind into Ukraine, whether under the guise of humanitarian convoys or any other pretext, without the express permission of the government of Ukraine," Hayden said.
She called Moscow's move "nothing but a pretext for further Russian escalation of the conflict," saying Russia was maintaining "a sizable military force" on the Ukrainian border capable of invading Ukraine "on very short notice".
"It has repeatedly fired into Ukrainian territory, and has sent an ever-increasing stream of military equipment and fighters into Ukraine," she added.
The clashes between Ukrainian government troops and pro-independence rebels in eastern Ukraine started in March and have claimed more than 2,000 lives.
Washington and its European allies have hit Moscow with sanctions, including measures against its financial and energy sectors.