The International Monetary Fund is losing confidence in Greece's ability to clean up its public finances and work off its mountain of debt, German magazine Der Spiegel reported on Saturday.

Citing an internal IMF memo, the news weekly said the body considered Greece's current readjustment programme insufficient and that new measures would have to be taken if the country is to avoid default and meet targets agreed with creditors.

The magazine said the IMF saw three options: either Athens enacts further austerity measures, private creditors write off more of their investments in the country's sovereign debt, or states in the euro zone increase bailout aid.

Earlier on Saturday, an adviser to Germany's finance minister Wolfgang Schaeuble told a Greek newspaper that a 50 percent write-down on Greek debt holdings, part of Greece's debt swap deal, is not enough to put the country on a viable path.

Banks and investment funds have been negotiating with Athens for weeks on the scheme, which aims to cut Greece's debt-to-GDP ratio from 160 percent to a more manageable 120 percent by 2020 -- a key part of the country's second, 130 billion euro (£107.1) bailout.

Der Spiegel, in its edition that hits news stands on Sunday, says this 120 percent target is now in question. It said the IMF had strong criticism for Athens' sluggish structural adjustment, especially regarding tax collection and state asset sales.

(Writing by Brian Rohan)