US President Barack Obama has blocked a Chinese-led group from purchasing several wind farms near a US naval station, saying the acquisition threatened the country's national security.According to US officials, the wind farm sites are all within or near restricted air space at an Oregon naval weapons system training base where pilotless drone aircraft are being tested.The President's move forces the company Ralls Corporation to divest its stake in the projects. It is the first time since 1990 that an American President blocked a foreign business deal.