Washington  - Pittsburgh has long evoked images of steel mills spewing smoke, but the Pennsylvania city has come a long way from its history as one of the United States' storied industrial centres.

The city has withstood the current global recession better than most the rest of the country, and President Barack Obama picked it to host the Group of 20 summit, which opens Thursday, in part because of Pittsburgh's dramatic shift away from a manufacturing economy in recent decades.

"Pittsburgh stands as a bold example of how to create new jobs and industries while transitioning to a 21st-century economy," Obama said a statement announcing the summit site earlier this year.

Natural resources from the surrounding mountains and Pittsburgh's location at the confluence of the Allegheny, Monongahela and Ohio rivers in far western Pennsylvania put it at the centre of the coal and steel industries beginning in the 1800s.

Writer James Parton described the sight of the city's smoke stacks in 1868 as "hell with the lid taken off." Today, he would likely not recognize the north-eastern city, which was ranked among America's cleanest by Forbes magazine in 2007 and was the top US city on The Economist's list of best places to live.

The Pittsburgh skyline is now dominated by skyscrapers housing banks and health-care companies.

The city's top employer is the University of Pittsburgh Medical Centre with 42,000 workers, followed by the university itself with 11,000, according to a local business group. Pittsburgh is also home to Carnegie Mellon University, known for its robotics and technology research, and 35 other colleges and universities.

All of the city's top 10 employers are now in health care, education, banking or telecommunications, even as its Forbes 500 companies still include once dominant US Steel and food company HJ Heinz, famous for ketchup and other condiments.

"Pittsburgh is truly a renaissance city that has experienced a great renaissance," Pennsylvania Governor Ed Rendell told reporters ahead of the Group of 20 summit.

The city's manufacturing economy collapsed in the middle of the last century, and the city has shrunk since then. Its population peaked at nearly 677,000 in 1950 and stands at about 335,000 today, US Census Bureau figures show. A total of 1.2 million people live in greater Allegheny County, which includes Pittsburgh.

Steel does remain a presence - if a much cleaner one - with the largest coke-producing facility in the country operated by US Steel in nearby Clairton and company headquarters still in downtown Pittsburgh.

Metal is immortalized in the name of the city's National Football League team, the Pittsburgh Steelers, winners of this year's Super Bowl championship. In ice hockey, the local Penguins won the Stanley Cup championship this year.

Pittsburgh's revival has been pointed to as a model of shifting an economy from manufacturing to a service and technology-based model. Rendell speculated that the city's focus on the environment and so- called green jobs was a chief factor in Obama's decision to hold the G20 summit there.

But the city has not been untouched by the global recession.

Unemployment stood at 7.7 per cent in July - a 13-year high, the Pittsburgh Tribune-Review reported, but still lower than the national rate now at 9.7 per cent. Recent statistics show the region continues to shed manufacturing jobs, but is actually picking up positions in education and health care.

"The national story is not necessarily the Pittsburgh story. For instance, foreclosures and steep housing losses are not something we've seen here," University of Pittsburgh economist Christopher Briem told the newspaper in June. "And unemployment has been lower than the national average for about two years."

The city's rebirth extends to the convention centre opened earlier this decade, where the G20 leaders will meet in a revitalized part of town, near a district of markets, restaurants and nightclubs.

Across the Allegheny River from the convention hall are the baseball and football stadiums and the Andy Warhol Museum, where leaders spouses will lunch during the summit. (dpa)