"At around noon today the OSCE Special Monitoring Mission (SMM) to Ukraine lost contact with some members of its Donetsk-based team. The team was west of Donetsk City on its way to Dnipropetrovsk when contact was lost," the Organisation for Security and Co-operation in Europe said in a statement.
The OSCE said the monitors were "detained".
"At around 19:00 contact was re-established when the group returned to Donetsk," it added.
It added that the observers included two Bulgarians and one each from Austria, the Netherlands, Finland, Italy, Norway, Poland, Russia, Slovakia and the US.
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"We are continuing with our efforts and utilising our contacts on the ground. The Ukrainian government, as well as regional authorities, have been informed of the situation," the statement continued.
Yesterday, the OSCE said it had lost contact with a first group of four observers.
An official in Vienna said the team -- a Dane, an Estonian, a Turk and a Swiss national -- was detained at a checkpoint in eastern Ukraine 40 minutes before contact was lost.
The Ukrainian foreign ministry has accused rebels of holding them, but the Turkish foreign ministry said the team appeared to be "safe and sound".
The Special Monitoring Mission currently has some 280 unarmed civilian members in Ukraine, whose job is to meet with local and national authorities as well as ethnic and religious groups and non-governmental organisations.