* Lawsuit filed in the U.S.
* U.S. Calamos claims EXIN has defaulted on loans
* Calamos and EXIN had agreed to buy National Insurance firm
* 718 mln euro deal was supposed to be finalised end March (Adds EXIN response)
By George Georgiopoulos
ATHENS, Feb 7 (Reuters) - A deal to sell National Bank of Greece's insurance unit to U.S. and Dutch investors looked fragile after a legal row erupted between the two buyers on Wednesday.
U.S. investor Calamos Family Partners (CFP) said in an emailed statement it had filed a lawsuit in the U.S. against Dutch EXIN Financial Services Holdings for defaulting on loans.
Calamos Investments and EXIN Partners, a Netherlands-based investor focused on insurance, reinsurance and asset management, had agreed last year to acquire 75 percent of the National Bank of Greece's insurance unit - National Insurance - for 718 million euros ($889 million).
EXIN declined to comment on the litigation itself, but said on its website it was 'fully committed' to seeing the deal succeed within the prescribed timeframe.
The deal was signed in June but the buyers have yet to notify regulators at Greece's central bank of key details.
"The Bank of Greece has not received the folder containing the final shareholding structure of the buying group," a senior central bank official told Reuters, declining to be named. "We are expecting EXIN to do so in the next days."
The investment was supposed to be concluded by the end of March according to the terms of the deal.
National Bank of Greece was not immediately available for comment. Under the deal, it was to keep a 25 percent stake in the insurance unit.
In an emailed press release, Calamos Family Partners (CFP) said it was seeking immediate repayment of more than $41 million in principal, plus interest, owed to them by EXIN under a series of loan agreements. The full amount was payable on December 31 2017, and was now in default.
It said EXIN has repeatedly failed to provide requested financial statements and other financial, accounting and regulatory information and records as required by the applicable loan agreements. In addition, the lawsuit seeks a full accounting of when and how EXIN used the borrowed funds.
EXIN later responded with a statement on its website.
"EXIN has recently been made aware that one of its partners has chosen to issue a press release regarding the existence of a litigation against EXIN. EXIN does not comment on litigation nor threats of litigation," the company said.
"EXIN remains fully committed to the expeditious close of the (National Insurance) transaction within the prescribed timeframe."
National Bank of Greece had put its insurance subsidiary up for sale as part of a restructuring plan approved by the European Union to exit non-banking operations and focus on core banking. (Reporting by George Georgiopoulos, editing by Louise Heavens and Elaine Hardcastle)