BlackRock Inc. has agreed to sell a 43-story office building in Singapore to Qatar Investment Authority, a sovereign wealth fund, for S$3.4 billion ($2.5 billion), in what the U.S. firm said was the largest-ever single-tower real estate deal in the Asia-Pacific region.

Asia Square Tower 1, located in the city-state's financial district, has over 1.25 million square feet of net rentable area and has Citigroup Inc. as its anchor tenant, BlackRock and Qatar Investment Authority said in a joint statement.

BlackRock was advised by real estate consultant firms JLL and CBRE.

"Following this flagship transaction, we expect there will be increasing investor interest in Singapore prime office stock in the coming months," Greg Hyland, head of capital markets Singapore at JLL, said in a separate statement.

The sale comes as vacancy rates in Singapore's office property sector are nearing their highest level in almost a decade, with the supply of commercial space set to increase amid slowing economic growth.

Developers are set to add 4 million square feet of office space in Singapore this year and another 1.4 million next year, said Nicholas Mak, executive director at SLP International Property Consultants.

BlackRock also owns a second tower in the Asia Square development.