Global premium segment growing for all three German luxury brands but Audi and BMW are inching ahead of Benz.

Mercedes-Benz has been trailing its fellow German rivals in global sales lately, having been surpassed recently by Audi too, and needs to increase capacity in order to catch up.

For the first six months of 2011, BMW sold 690,000 units world-wide. For Audi, sales for the first half of the year were 653,000, while Mercedes-Benz came in third with 611,000.

According to Daimler CEO Dieter Zetsche, catching up is a matter of building more cars.

The German premium brand has invested more than $2 billion at its U.S. plant in Tuscaloosa, Alabama in order to boost capacity. The plant is where the M-, R- and GL-Class vehicles are assembled.

While both automakers have increased their forecasts for the year, BMW is still well ahead. Mercedes-Benz forecasts sales for 2011 of 1.35 million units. BMW currently forecasts its sales at 1.6 million for the year.

All three German premium brands have seen global demand increase lately, particularly in the still fast-growing market in China.

Gallery: Mercedes needs to build more cars to keep up with Audi, BMW