Secretary General Lamberto Zannier met Ukrainian Foreign Minister Andriy Deshchytsia, who demanded the "immediate release" of the seven Europeans detained in the flashpoint eastern Ukrainian town of Slavyansk.
"We are in contact with local authorities as well as with other international monitors," the minister said.
Zannier declined to give an update on the fate of the group -- including four Germans, one Dane, one Czech and one Pole -- telling reporters only that "we are this moment discussing what to do".
Rebels holding the group have previously described them as "prisoners of war" and said they will only be freed in exchange for the release of militants in custody.
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They are also holding four Ukrainians, whose fate is also unclear.
Eight of the European monitors appeared before the cameras on Sunday and one of them said they were in good health.
One of the men, a Swede, was later freed on medical grounds as he suffers from diabetes.