An inquiry is being held in Berlin into the circumstances surrounding a Nato air strike in Afghanistan in which up to 142 people were killed.
Afghan civilians were among the victims of the airstrike in Kunduz in early September, which was carried out on the orders of a German commander.
Defence Minister Karl-Theodor zu Guttenberg is facing calls to quit.
Opponents accuse the German government of a cover-up over the strike on two fuel tankers hijacked by the Taliban.
Mr Guttenberg - who held a different post in government at the time of the attack - has described the airstrike as "the most important event in the military history of post-war Germany".
He is under growing pressure from opposition parties to resign over the strike, amid allegations that Chancellor Angela Merkel's government withheld information and covered up the scale of civilian casualties.
Mr Guttenberg initially described the strike on two fuel tankers hijacked by the Taliban as "militarily appropriate".
But he recently changed his mind, saying the attack was "militarily inappropriate", claiming he now had access to new military reports.
The political fallout from the air strike has already led to the resignations of former Defence Minister Franz Josef Jung and the army chief of staff.
Opposition parties are now calling for Mrs Merkel to appear before the parliamentary committee to answer questions about her role in the affair - and they want Mr Guttenberg to testify.
They might be summonsed by the committee in January, just before the international conference on Afghanistan - which will be held in London - where Germany is likely to come under more pressure to send extra troops to the region.
Christine Buchholz, a member of the opposition Left Party, said the government has been "constantly lying" on the war in Afghanistan and the Kunduz strike.
"First of all they denied that civilians we killed - we now know that a big number of civilians were killed," she said.
"Secondly, they said the fuel tankers were the targets of the bombings - we now know the target was to hit the Taliban fighters and this is absolutely against the Afghanistan mandate."
The German parliament recently extended the Afghan mission for another year - but it is still deeply unpopular at home.
According to a recent poll, most Germans want their troops to withdraw from Afghanistan as quickly as possible.
The controversy over the Kunduz air strike has left the government on the defensive.