Coalition talks have broken down between German conservative leader Angela Merkel and the Greens.
The Green leaders quizzed Mrs Merkel about her ecological policies
The Greens leadership said policy differences between them and Mrs Merkel's Christian Democratic Union (CDU) were too wide to be bridged.
Mrs Merkel, who won a slender victory in Sunday's inconclusive elections, acknowledged the gulf.
Correspondents say the lack of progress could lead to a grand alliance between the CDU and Gerhard Schroeder's SPD.
Chancellor Schroeder and Mrs Merkel held exploratory discussions on Thursday - but each claims the right to lead a new government.
They are due to meet again next week but negotiations between all the parties could continue for weeks.
After talks with the Greens on Friday, Mrs Merkel told reporters: "There was of course great difficulty in finding areas of agreement. We do not foresee another round of talks for the moment."
The Greens' co-president Reinhard Buetikofer said the CDU could not explain whether they would drop their "neo-liberal, radical market, anti-ecological policies" and therefore further talks were "futile", according to AP.
1. Christian Democrats/Christian Social Union (CDU/CSU): 225
2. Social Democrats (SPD): 222
3. Free Democrats (FDP): 61
4. Left Party: 54
5. Greens: 51
Meanwhile, a senior German official in charge of the archive of the East German secret police, or Stasi, claims at least seven of the new MPs elected on Sunday used to work unofficially for the force.
Marianne Birthler told the Mitteldeutsche Zeitung newspaper that the seven were all members of the left-wing Die Linke - Left party - which is due to get 54 seats in the new parliament after winning nearly nine per cent of the vote.
She has called for all new German parliamentarians to face obligatory background checks, which should be made public.
The Left party is a mix of disgruntled former Social Democrats and former East German communists.
One of its co-leaders, Gregor Gysi, has often faced and denied allegations that he worked for the Stasi. Earlier this month he secured a court order to stop his file being made public.
The Left has been shunned by both the SPD and CDU in the efforts to form a new coalition this week.
This means there is a bloc of 54 MPs in the Bundestag who are not available for building a new coalition - which is why the other parties are finding it so difficult to form a new government, our correspondent says.