Speaking to reporters after talks with Swiss counterpart Didier Burkhalter, Steinmeier said he could not elaborate on efforts to free the men, given the "highly sensitive stage of the ongoing negotiations".
   
Neutral Switzerland is currently at the helm of the Organization for Security and Cooperation in Europe, which sent the observers to Ukraine to monitor a peace deal reached in Geneva last month that has been rejected by the pro-Russian rebels.
   
The Swiss government has been striving to defuse the Ukraine crisis, which has raised Western tensions with Russia to their sharpest since the Cold War.
   
"Our goal is the unconditional release of the hostages in Slavyansk," said Burkhalter.
   
Eight OSCE observers, four of whom are Germans, were captured last Friday in the flashpoint town of Slavyansk in mainly Russian-speaking eastern Ukraine.
   
They were presented to the media Sunday as "prisoners of war" in what Germany said was a "repugnant" display.
   
Berlin has been pushing Russian President Vladimir Putin to persuade the pro-Moscow rebels to free the observers.
   
One of the hostages, a Swede who was said to suffer from diabetes, was freed late Sunday, but the Germans, a Pole, a Dane and a Czech remain in custody.
   
The rebels also captured four Ukrainian OSCE representatives, but they have not been seen in public since.
   
Steinmeier called the Ukraine crisis a "huge challenge for all concerned", saying clashes on Friday showed the violence in the embattled country was far from over.
   
Burkhalter said there were plans to launch a new drive to resolve the crisis and that high-level contacts were under way, but did not elaborate.