BERLIN, Aug 19 (Reuters) - The Free Democrats (FDP), who could form a coalition with Chancellor Angela Merkel's conservatives after an election next month, want the next government to agree a plan to pull troops out of Afghanistan.

Juergen Koppelin, an FDP member with responsibility for defence issues, told Germany's Bild newspaper that the NATO mission in Afghanistan lacked a clear strategy and had produced too many victims.

"The next government must formulate a precise plan that spells out how a pullout of the German army over the coming years would look," Koppelin said.

"Our soldiers operating in Afghanistan and their families need to know that the mission will end," he added.

The comments are significant because the FDP, a centre-right, pro-business party, is expected to wield influence over foreign policy if it is able to form a ruling partnership with Merkel's conservatives.

Its leader, Guido Westerwelle, is expected to become foreign minister if such a coalition emerges after the Sept. 27 vote.

The stance puts the FDP at odds with Merkel's Christian Democrats (CDU), who have refused to fix a timeline for a pullout. Defence Minister Franz Josef Jung has said German troops could stay another five to 10 years.

Germany has about 4,200 troops in Afghanistan, where violence has surged in the run-up to a presidential election on Thursday.

On the eve of the vote, gunmen stormed a bank building in Kabul and battled police for hours in what was the third major attack in the Afghan capital in the past five days.

The mission is highly controversial in Germany, with polls showing close to two-thirds of Germans oppose it, but it has not been a major issue in the election campaign, in part because the two biggest parties -- the CDU and Social Democrats (SPD) -- agree the troops must stay.

A parliamentary mandate, which allows Germany to contribute up to 4,500 troops to the NATO mission, expires in December and must be renewed if the troops are to remain in Afghanistan.

(Writing by Noah Barkin; editing by Robin Pomeroy)