Greece's fiscal problems must be solved within the euro zone rather than with the International Monetary Fund (IMF), French Economy Minister Christine Lagarde said in an interview published in a Greek newspaper on Sunday.

Asked if it might be more effective for Greece to call in the IMF rather than possibly borrowing money from European Union countries, Lagarde told To Thema newspaper: I consider that the collective will of all of us is to settle the issue within the euro zone.

Germany's finance ministry has sketched out a plan in which countries using the euro currency would provide aid worth between 20 billion and 25 billion euros ($27-$33.7 billion) for Greece, German weekly Der Spiegel reported on Saturday.

Citing initial considerations by the ministry, Der Spiegel said the share of financial aid for Greece would be calculated according to the proportion of capital each country holds in the European Central Bank.

Lagarde did not refer to any such plan in the interview. She said Greece must be ready to take additional measures if its current austerity plan does not prove to be enough to reduce its budget deficit by 4 percentage points this year.

If it is necessary, Greece will have to take additional measures, Lagarde said in the interview.

Greece has pledged to reduce its budget deficit by 4 percentage points to 8.7 percent of gross domestic product (GDP) in 2010.

Greek Prime Minister George Papandreou said earlier today in an interview with BBC television Athens so far was meeting the goals it set out in its austerity plan and Greece's borrowing needs were covered until mid-March.

Asked about comments she made earlier this week about the possible role of some banks in Greece's fiscal problems, Lagarde told the newspaper: What I have said is that observing the fluctuations of CDS, someone might assume that there is a speculation game in the background.

We know that this (the speculation) is happening. I don't know who they are ... but they are not only Anglo-Saxons, Lagade said.

(Reporting by Lefteris Papadimas; editing by Ingrid Melander and Karen Foster)