"We want to help Greece and we will help Greece. But Greece has to want to be helped. If they deviate from the agreed reform path, then the payment of further tranches of aid is not possible," Guido Westerwelle told lawmakers.
"We are sticking to our pledges to help. But that means as well that the agreed reforms in Greece must be carried out.
"We want to keep the eurozone together. The future of Greece in the eurozone now lies in the hands of Greece," he stressed.
Greek politicians are battling to form a government after inconclusive elections at the weekend that handed massive gains to parties opposed to austerity measures the EU has attached to its bailout packages.
There were small signs of progress late on Thursday after socialist leader Evangelos Venizelos won the conditional support of the small Democratic Left (Dimar) party in negotiations to form a cabinet.
The stalemate in Greece has raised concerns about possible political chaos that could kill off reforms and eventually force the debt-laden nation to leave the eurozone.
Germany has kept up the pressure on Greece to push through the austerity measures required or face the consequences.
"Solidarity is not a one-way street," Westerwelle cautioned.
Westerwelle's comments followed remarks by Finance Minister Wolfgang Schaeuble, who appeared to suggest that the eurozone could cope if Greece left the 17-nation club.
Asked by the regional Rheinische Post daily whether the eurozone could withstand a Greek exit, the minister said: "Europe won't sink that easily."
"We want Greece to remain in the eurozone. But it also has to want this and to fulfil its obligations. We can't force anyone.
"We have learned a lot these past two years and have built protection mechanisms. The danger of contamination for other countries in the eurozone has become weaker and the eurozone as a whole has become more resistant."
"No-one is threatening anyone here," Schaeuble said in the interview. "But we must be honest... and tell our Greek friends and partners that there is no other way than the one that we have chosen together."
Earlier in the week, the eurozone announced it was blocking 1.0 billion euros ($1.3 billion) out of 5.2 billion euros in bailout loans for Greece until Monday amid uncertainty over the country's political future.
Germany's chancellor, Angela Merkel, said on Monday that reforms in Greece were "of utmost importance."