UPDATE 1-Germany seeks explanation of KfW transfer to Lehman

(Adds S&P on no rating change)

BERLIN, Sept 17 (Reuters) - Germany's Finance Ministry wants an explanation as to why state lender KfW transferred 300 million euros ($426 million) to Lehman Brothers LEH.N on the day the U.S. bank filed for bankruptcy, a ministry spokesman said on Wednesday.

"What we have had to read today is astonishing and exasperating," Finance Ministry spokesman Torsten Albig said when asked about reports of the transfer.

"We expect a swift explanation of such a technical failure, which is inexplicable to us, but which will hopefully be explained soon," he said at regular government news conference. KfW said on Tuesday it mistakenly transferred the funds to Lehman on the day the investment bank filed for bankruptcy. The Frankfurter Allgemeine Zeitung (FAZ) newspaper reported that the payment was due to an erroneous swap payment.

Ratings agency S&P said KfW's Lehman exposure had no impact on the German bank's credit rating.

"KfW's extremely strong capacity to meet its financial commitments continues to be supported by the implicit and explicit support from the Federal Republic of Germany (AAA/Stable/A-1+)," S&P said in a statement.

KfW has already taken a hit from liquidity lines given to business lender IKB (IKBG.DE), the country's biggest casualty of the subprime crisis. KfW, IKB's main shareholder, plans to sell the subprime casualty to U.S. investor Lone Star [LS.UL]. (Reporting by Paul Carrel and Erik Kirschbaum)