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 that 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