Uber, the startup known for its on-demand car service, is looking to raise $500 million at a $12 billion valuation, according to a report byThe Wall Street Journal. The report says that Uber is talking to investors at BlackRock (BLK), General Atlantic and Technology Crossover Ventures.

Investors have been asked to convey interest by the end of the week, with the deal expected to close by the end of the second quarter. Bloomberg previously reported that Uber was looking to raise financing at a $10 billion valuation.

The San Francisco-based company has already raised $405 million at a $3.8 billion valuation. Existing investors include Goldman Sachs (GS), Google Ventures (GOOG), and Jeff Bezos.

Founded by CEO Travis Kalanick in 2009, the mobile application has popularized car service. Instead of planning ahead or looking for a taxi, users can use the location-based app to find a nearby car to arrive within minutes.

As a result of the increased competition for taxi services, Uber has faced government obstacles in various metropolitan areas. Boston taxi drivers held a rally Thursday to try to “shut down” Uber.

Uber recently launched a delivery service in New York City called UberRUSH. Couriers can pick up items on-demand and bring them to a nearby location.
 

Follow Katie Roof on Twitter @Katie_Roof