"We want to do everything we can to help Greece ... we can help but we are not going to pour money into a bottomless pit," Wolfgang Schäuble told SWR radio.
"We have always said that all conditions must be fulfilled before we can take final decisions and that the time was pressing. I have doubts that all conditions have been fulfilled," added the minister.
He took aim at Greece's conservative Nea Demokratia party, which enjoys a huge lead in the polls ahead of elections expected in April. Their lead is "exactly the problem," Schäuble said.
"I am not certain that all political parties in Greece are aware of their responsibilities given the difficult situation in which their country finds itself," the minister said.
"When you look at the situation in domestic Greek politics ... the question is: who is going to guarantee that what we decide now will also be valid after the elections?" asked Schäuble.
Late Tuesday, Luxembourg Prime Minister Jean-Claude Juncker, who is head of the eurozone finance ministers, scrapped a planned meeting on more aid, opting for a conference call instead as Greece failed again to meet EU demands.
"I did not yet receive the required political assurances from the leaders of the Greek coalition parties on the implementation of the programme," Juncker said in a statement.
Further "technical work" needed to be done, Juncker added.
Greece desperately needs the €230-billion ($303-billion) rescue package - €130 billion in fresh loans and a €100-billion write-down on privately-held bonds - to avoid defaulting on €14.5 billion in debt owed on March 20.