* Air Berlin aircraft grounded at Keflavik airport
* Air Berlin filed for insolvency in August
* Airport operator cites charges from before the insolvency
* Air Berlin says grounding is "unlawful" and "unacceptable" (Adds statement from Air Berlin)
FRANKFURT/BERLIN, Oct 20 (Reuters) - An Air Berlin airliner was grounded at Iceland's Keflavik airport late on Thursday because the insolvent German carrier had not paid its airport charges, Keflavik operator Isavia said in a statement.
It said the unpaid charges had been incurred before Air Berlin, which has struggled to turn a profit over the last decade, filed for insolvency on Aug. 15.
According to German magazine Stern, the Airbus A320 landed at Keflavik at 10:35 p.m. local time on Thursday and was due to depart for Duesseldorf just after midnight.
Air Berlin said the grounding was "unlawful" and "unacceptable".
"We have told Keflavik airport repeatedly that any outstanding demands of payment relating to the time before Aug. 15 must be registered on the insolvency table due to insolvency law," Air Berlin said in a statement.
"We have called on Keflavik airport to cease its unlawful actions immediately," it added.
Air Berlin's flights have been kept aloft by a government loan since the airline's insolvency filing, giving the carrier time to negotiate with prospective buyers for its assets. It has said flights will cease by Oct. 28 at the latest.
Lufthansa has agreed to buy large parts of Air Berlin. Talks with other possible buyers including Britain's easyJet are continuing. (Reporting by Maria Sheahan and Christian Kraemer; editing by Jason Neely, Greg Mahlich)