LONDON, July 1 (Reuters) - German funds firm Union Investment Real Estate said on Tuesday it had bought the Fuerstenhof office building in Frankfurt's financial district for 129 million euros ($203.6 million).

Union said the vendor was Eurocastle Investment Ltd (ECT.AS: Quote, Profile, Research), a Guernsey-registered company backed by U.S. hedge fund and private equity firm Fortress Investment Group LLC (FIG.N: Quote, Profile, Research).

The 108 year-old Fuerstenhof building near the European Central Bank headquarters comprises 18,450 square metres (198,600 sq ft) of office space and was fully let on a long-term lease to Dresdner Bank, Union said.

In an earlier statement, Eurocastle said it had completed two property sales for a total of 219 million euros or 99 percent of their first-quarter net asset value, without giving any further details.

Eurocastle could not immediately be reached for comment. (Reporting by William Kemble-Diaz; Editing by David Holmes) (See www.reutersrealestate.com for the global service for real estate professionals from Reuters).