US real estate investor Blackstone Group is buying the iconic 110-story Willis Tower in Chicago, the second-tallest building in the United States.

Blackstone gave no price for the purchase from the current owners, 233 South Wacker LLC, but various US media reports put the deal at $US1.3 billion ($A1.7 billion). Known for most of its existence as the Sears Tower, the 442-metre building was the tallest building in the world for a…