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 quarter-century after its completion in 1973.

Today it ranks tenth, according to The Skyscraper Center.

The title of the tallest building in the United States went to One World Trade Center in New York, completed two years ago.

“We are delighted to be acquiring this iconic building on behalf of our limited partners,” said Jacob Werner, a Blackstone real estate group managing director.

“We are bullish on Chicago as companies expand within and move into the city and look for first-class office space.”

The purchase adds 350,000 square metres of space to New York-based Blackstone’s huge investment portfolio of real estate and related businesses around the world, including Hilton Worldwide, Invitation Homes, and Chinese shopping malls owner SCP.

Blackstone says in all it has $US81 billion in investor capital under management.