Philadelphia ? At City Hall, leaders of the nation’s sixth-largest city are warning about painful cuts as the recession slices deeper and deeper into the budget: library cuts, pool closures, less snow removal and layoffs.

But on an empty lot six blocks away, private developers are pushing ahead with plans to build a 1,510-foot skyscraper that would be one of the tallest buildings in the world.

In the midst of the worldwide financial crisis, the developers of the American Commerce Center envision a massive construction project that would employ hundreds of idled construction workers — and, by 2012, an iconic structure that would funnel many more jobs into the city.

But like a slew of sky-high building projects in the works from Chicago to Dubai, the building faces huge challenges: Credit is hard to find and potential tenants are gun-shy.

The Philadelphia City Council has already approved zoning changes to clear the way for the $1.1 billion project, which includes plans for a 63-floor office tower, along with a hotel, retail, parking and other amenities downtown.