China is building up several unoccupied land features in the South China Sea, according to Western officials, an unprecedented move they said was part of Beijing’s long-running effort to strengthen claims to disputed territory in a region critical to global trade.

While China has previously built out disputed reefs, islands and land formations in the area that it had long controlled — and militarized them with ports, runways and other infrastructure — the officials presented images of what they called the first known instances of a nation doing so on territory it doesn’t already occupy. They warned that Beijing’s latest construction activity indicates an attempt to advance a new status quo, even though it’s too early to know whether China will seek to militarize them.

Fishing fleets that operate as de facto maritime militias under the control of authorities in Beijing have carried out construction activities at four unoccupied features in the Spratly Islands over the past decade, according to the officials, who asked not to be identified to discuss sensitive information. Some sand bars and other formations in the area expanded more than 10 times in size in recent years, they said.