An Ivanhoe Cambridge Inc. venture agreed to buy a Manhattan office tower from Blackstone Group LP for about $2.25 billion, extending a surge of demand for prime U.S. properties, two people with knowledge of the deal said.

The Ivanhoe group signed a contract yesterday to purchase 1095 Avenue of the Americas, a 42-story building on Bryant Park, said the people, who asked not to be named because the transaction is private. The sale would be the largest of an entire U.S. property since a group led by Boston Properties Inc. bought the General Motors Building in New York for $2.8 billion in 2008, according to Real Capital Analytics Inc.