Vietnam has been ramping up its dredging and landfill work in the Spratly Islands in the South China Sea, creating another 330 acres of land since December last year, a U.S. think tank said in a report.

The expansion, much larger than 120 acres it had created between 2012 and 2022, made Vietnam second only to China in terms of island buildup in the Spratly Islands, Washington's Center for Strategic and International Studies (CSIS) said in the report this week, citing satellite imagery.

Vietnam's Ministry of Foreign Affairs did not immediately respond to a request for comment.

China claims sovereignty over vast swathes of the South China Sea, including the areas where Vietnam has been building up islands.

The sea is one of the world's most contested waterways, where more than $3 trillion of trade passes each year. China, Brunei, Malaysia, the Philippines, Taiwan and Vietnam have lodged competing claims for some or all of the Spratly Islands.

The recent expansion by Vietnam was the most noticeable at Barque Canada Reef, called Bai Thuyen Chai in Vietnam, where the report, produced by CSIS's Asia Maritime Transparency Initiative (AMTI), said 210 acres of land was created in the past year.

The report said Vietnam had started using cutter suction dredgers to "accelerate its dredging efforts", and last month, it began dredging at two additional features, including South Reef and Central Reef.

It said Vietnam's efforts have remained focused on dredging and landfill, with construction of infrastructure yet to begin.

China created more than 3,200 acres of land from 2013 to 2016 in the area, the think tank said.

In August, the Philippines, Malaysia, Taiwan and Vietnam rejected a map released by China that denotes its claims to sovereignty including in the South China Sea.