The statement was released hours after the Pentagon said that Russian fighter jets had made about half a dozen incursions into Ukrainian airspace over the previous 24 hours.

Col. Steven H. Warren, a Pentagon spokesman, said that Gen. Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, had spoken with his Russian counterpart on Thursday, but offered no details.

Also Friday, a group of foreign military observers traveling under the auspices of the Organization for Security and Cooperation in Europe, along with their Ukrainian hosts, were detained by pro-Russian separatists in Slovyansk, the separatists and the Ukrainian government said.

The government said seven foreign observers and five Ukrainian military officers had been seized. The detention appeared to give the rebels, who on Thursday had released an American journalist held for three days, a new set of foreign prisoners.

Unconfirmed early reports had identified the detainees as monitors from the security organization, but the organization’s officials said all of their permanent staff members and monitors had been accounted for. Tatyana Baeva, a spokeswoman in Vienna, said the reports evidently referred to a separate German-led group of unarmed military inspectors.