SLOVYANSK, Ukraine — Antigovernment militants in eastern Ukraine on Saturday rebuffed international calls for the release of a group of European military observers, but suggested that they would consider a prisoner exchange.

The military observers — a group of officers reportedly from Germany, Poland, Sweden, the Czech Republic and Denmark — were detained on Friday at a rebel checkpoint at the edge of this city while traveling with a Ukrainian military delegation, which was also held.

The observers, known as a military verification team, had been working under the auspices of a document from the Organization for Security and Cooperation in Europe that allows member nations — Ukraine, in this case — to invite other member nations to send observers to examine security conditions.

The militants have accused the observers of espionage.

Early Sunday morning, a spokesman for the O.S.C.E. in Kiev, the Ukrainian capital, said members of its special monitoring mission to Ukraine, a set of monitors distinct from the detained military observers, would travel to Slovyansk from Donetsk to meet with the rebel authorities. “We’re looking forward to access Slovyansk as soon as possible to play a role in the resolution,” said the spokesman, Michael B. Bociurkiw.