Chrysler Group LLC will invest $600 million in its Belvidere  assembly plant to build new cars starting in 2012, the company announced Thursday.

The investment won’t create any new jobs, but the company said it will retain the 2,349 jobs  at the plant and a nearby parts stamping plant.

Work began last summer on a new 638,000-square-foot body shop, where body side panels, fenders and other parts are welded together, and Chrysler is installing new machinery. Completion is scheduled for sometime in 2011.

Company spokeswoman Jodi Tinson would not say what new vehicle the plant will build. But it’s likely to be a replacement for the factory’s current products, Dodge Caliber and Jeep Compass and Patriot crossovers.

Chrysler CEO Sergio Marchionne has said the company intends to build a 40 mile-per-gallon small car for the U.S., in addition to the Fiat 500, which will go on sale in December.

The company has had poor-selling small cars, but Chrysler hopes the 500 will change that when it arrives at selected dealers across the U.S. The 500 is being built in Mexico.

The Belvidere complex is about 70 miles northwest of Chicago along Interstate 90 near the Wisconsin-Illinois border. The factories, which began operation in 1965, have built models such as the Town & Country station wagon, and the Neon and Omni compacts.

Chrysler says that with the Belvidere expansion, it has invested $2.1 billion in its U.S. facilities since leaving bankruptcy protection in June of last year.