China has dramatically ramped up its land reclamation efforts in the South China Sea in the past five months, building artificial islands at an unprecedented pace to bolster its territorial claims in the area, US defense officials said Friday.

"China has expanded the acreage on the outposts it occupies by some four hundred times," said a defense official, adding Beijing has reclaimed 2,000 acres (800 hectares) since January 2014  -- and 75 percent of the total this year alone.