"I can confirm that on several occasions in the last 24 hours, Russian aircraft have entered Ukrainian airspace," the spokesman, Colonel Steve Warren, said.
The disclosure came a day after the Pentagon sharply criticized Russia's announcement of new military drills near the border with Ukraine. U.S. Defense Secretary Chuck Hagel called Russian military activity along the border "dangerously destabilizing" and provocative.
The Pentagon confirmed Russian reports that the top U.S. military officer, General Martin Dempsey, had spoken on Thursday by telephone with Russian General Valery Gerasimov, the chief of the general staff. It gave no details of the conversation.
Russia's Interfax news agency said Gerasimov told Dempsey that Ukraine had deployed a "substantial group of forces" near the Russian border, including troops whose purpose was to conduct sabotage.