Gernot Erler said Tbilisi had breached a 1992 ceasefire agreement struck with Russia over the renegade Caucasus enclave, monitored essentially by Russian peacekeepers.
"In this sense, it is also a question of a violation of international law as soon as you start to go down the road of military action," Erler told German radio station NDR Info on Saturday.
Erler acknowledged prior provocation of the Georgian leadership from Russian-backed South Ossetia's separatists, but said he understood Russia's reaction.
While South Ossetia remains sovereign Georgian territory, Russia has taken over the role of providing economic support to the region, Erler noted.
From a Russian perspective, peacekeeping troops have therefore been attacked.
"It's an insane, bloody war which will surely do nothing to resolve the problem of this separatist Ossetian province," added Erler, deputy to Foreign Minister Frank-Walter Steinmeier.
Steinmeier himself called for the Georgians and Russians to pull back their troops, warning of the "danger of a perilous fire" which could engulf the Caucasus region.
"It is nothing less than the threat of war.... We are facing the threat of a dangerous fire" that could spread in the region, Steinmeier said in an interview with the Bild am Sonntag newspaper.
Several more members of their Social Democratic party, as well as the opposition Greens, have adopted a critical attitude towards Tbilisi, in contrast to Chancellor Angela Merkel's conservatives.
Merkel is due to meet with Russian President Dmitry Medvedev at the Russian Black Sea resort of Sochi, not far from the border with Georgia's breakaway region of Abkhazia, on Friday.
Georgia declared a "state of war" on Saturday as Russia bombed the country and their armies battled for control of the separatist region of South Ossetia.
Alongside another former Soviet republic, Ukraine, Georgia was prevented from obtaining candidate status for NATO membership at an alliance summit earlier this year.