Nokia said today it will lay off 4,000 employees at three factories as it shifts device assembly to Asia, where the majority of its component suppliers are based.

The plants in Komarom, Hungary; Reynosa, Mexico; and Salo, Finland will focus instead on smartphone product customization for customers mainly in Europe and the Americas.

“Shifting device assembly to Asia is targeted at improving our time to market,” Nokia Executive Vice President Niklas Savander said in a statement. “By working more closely with our suppliers, we believe that we will be able to introduce innovations into the market more quickly and ultimately be more competitive.”

Savander said Nokia recognized that the “planned changes are difficult for our employees” and reiterated the company’s ongoing pledge of “supporting our personnel and their local communities during the transition.”

Nokia President and CEO Stephen Elop warned last year that the company planned to reorganize factories in Finland, Hungary and Mexico.

The company has closed plants and restructured several of its divisions to streamline operations for its transition from its legacy Symbian platform to Microsoft’s Windows Phone.

The latest round of job cuts comes after Nokia said in September it would eliminate 3,500 positions by closing a factory in Cluj, Romania and restructuring its location division. The company cut 7,000 positions earlier in 2011 by transferring its Symbian business to Accenture and terminating employees in Denmark, Finland and the United States.

The downsizing is aimed at reducing Nokia’s operating expenses by $1.5 billion through 2013.