Uniper, hit by the cost of trying to replace reduced Russian gas deliveries, struck an initial 15 billion euro ($14.9 billion) bailout deal with the German government in July, including a 9 billion euro credit line from KfW, 5 billion of which has already been drawn.

That line could be increased as a backstop measure to account for a much more significant increase in gas prices than was expected at the start of the bailout talks in June, the sources said.

Details of the bailout are still under discussion and a term sheet is expected to be signed in mid-September, the people said, adding other options were also under discussion and that KfW may not need to increase the line.

Germany's Economy Ministry currently sees no need to adjust the Uniper rescue package, a spokesperson said, adding it was unclear how the situation would develop going forward.

KfW declined to comment and Uniper was not immediately available for comment.

Uniper has been forced to buy gas in the spot market where prices have soared to replace a shortfall in deliveries from Russia, which has reduced flows through the Nord Stream 1 pipeline since mid-June.

Uniper's gas losses stood at 3.8 billion euros as of Aug. 17 and, according to comments by finance chief Tiina Tuomela last week, are rising by roughly 100 million euros each day.

At the current rate this will amount to 8.2 billion euros by Oct. 1 when a gas levy kicks in that allows Uniper to pass on most of the higher gas costs to customers and massively cut losses.

That would be higher than a 7 billion euro threshold beyond which the German government has pledged to backstop losses, although the mechanism by which it would do so still needs to be clarified, the sources said.

They added that increasing the gas levy could also help cushion the blow.

"At the end of June, when we started the negotiations, we expected the backstop to not be reached at all," Uniper Chief Executive Klaus-Dieter Maubach told journalists last week after presenting a 12.3 billion euro first-half loss.

Maubach said the worst-case assumption at the time had been that the backstop threshold would be reached sometime after the gas levy kicks in. "But now it's clear that it will be reached earlier."

($1 = 1.0059 euros)

(Additional reporting by Andreas Rinke, editing by Kirsten Donovan)

By Christoph Steitz