Blackstone Group LP, the largest private-equity firm, agreed to buy a majority stake in 40 U.S. shopping centers from UBS AB property funds in a deal valued at about $1.1 billion, as it expands its retail holdings.

Kimco Realty Corp., UBS’s joint-venture partner on the properties, will increase its ownership to 33 percent from 18 percent, David Henry, the retail landlord’s president and chief executive officer, said Wednesday on a conference call with investors. The venture has about $692 million in debt on the properties, according to a filing by New Hyde Park, New York-based Kimco. The portfolio is 5.7 million square feet and was 96.1 percent leased at the end of 2012.