German business confidence has brightened unexpectedly this month, offering hope that a slowdown in the euro zone’s “core” economies would prove modest despite the threat of Greece’s debt crisis.

The Munich-based Ifo Institute reported its business climate index had risen for the first time since February. German businesses’ assessment of conditions improved sharply to hit the highest level since the survey started in 1991. Optimism for the next six months fell to the lowest for more than a year, however.

The result suggested German companies had shrugged off worries about debt crises, higher oil prices and fiscal austerity.