In the Spratly Islands, Hanoi added about 534 acres over the past year, taking its total reclaimed area to roughly 2,771 acres, a new report said.
The Asia Maritime Transparency Initiative said the expansion comes as Vietnam and China intensify land reclamation to strengthen rival territorial claims.
Although Vietnam had appeared to narrow the gap with China early last year, Beijing has since widened its lead in reclaimed territory.
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With new military outposts rising from the sea, how close is the South China Sea to open conflict?
As artificial islands destroy vital coral reefs, is international environmental law failing in the South China Sea?