It is rare for China to give such detail about its plans for the artificial islands. The rapid reclamation taking place on seven reefs has alarmed other claimants and drawn U.S. criticism, including from Defence Secretary Ash Carter, who is visiting Japan and South Korea this week.
"The relevant construction is a matter that is entirely within the scope of China's sovereignty. It is fair, reasonable, lawful, it does not affect and is not targeted against any country. It is beyond reproach," Hua added.
China claims most of the South China Sea, through which $5 trillion in ship-borne trade passes every year. The Philippines, Vietnam, Malaysia, Brunei and Taiwan also have overlapping claims.
All but Brunei have fortified bases in the Spratlys, which lie roughly 1,300 km (800 miles) from the Chinese mainland but much closer to the Southeast Asian claimants.
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While China's new islands will not overturn U.S. military superiority in the region, workers are building ports and fuel storage depots and possibly two airstrips that experts have said would allow Beijing to project power deep into the maritime heart of Southeast Asia.
Asked about Hua's comments, U.S. State Department spokesman Jeff Rathke called the land reclamation "destabilizing" and said it was "fueling greater anxiety within the region about China's intentions amid concerns that they might militarize outposts on disputed land features in the South China Sea."
"We very much hope that China would recalibrate in the interests of stability and good relations in the region," he told reporters in Washington.
Western and Asian naval officials privately say that China could feel emboldened to try to limit air and sea navigation once the reclaimed islands are fully established.
The U.N. Convention on the Law of the Sea does not legally allow for reclaimed land to be used to demarcate 12-nautical-mile territorial zones, but some officials fear China will not feel limited by that document and will seek to keep foreign navies from passing close by.
Jin Crannog, associate dean of the School of International Studies at Beijing's Renaming University, said that China probably felt it needed to give its side of the story following growing criticism from Washington over the reclamation.
"The motivation in giving an explanation is a good one, to set minds at ease," Jin said.
Hua's comments came hours after a Washington-based think tank published new satellite images that show China is quickly reclaiming land around Mischief Reef in the Spratlys within an area the Philippines regards as its exclusive economic zone.