Vietnam has significantly expanded its outposts in the contested Spratly Islands during the past six months according to a report released Friday by the Asia Maritime Transparency Initiative (AMTI).

AMTI, a Washington-based think tank at the Centre for Strategic and International Studies, found that Vietnam has created “almost as much new land” in the islands over the past six months as it had in the previous two years combined. 

While Vietnam’s dredging and landfill was a mere one-tenth of China’s total just three years ago, Hanoi’s conduct is currently stretched to 2,360 acres and amounts to nearly half of China’s 4,650 acres. The increase marks a major change in Vietnam’s presence in the disputed Spratly Islands.

The three largest outposts in the Spratly Islands are the Mischief, Subi, and Fiery Cross reefs, all of which belong to China. Vietnam, however, owns the next four largest outputs, named Namyit Island, Pearson Reef, Sand Cay, and Tennent Reef. AMTI referred to these four outposts as the “newly expanded Vietnamese reefs” in its report. Along with dredging and landfills, Vietnam has also started to build new construction and facilities across its outposts, including the erection of a harbor at the Central Reef.

The Spratly Islands are the subject of an ongoing territorial dispute concerning the ownership of the set of islands and various maritime features including reefs and banks. The islands are situated in the South China Sea among China, the Philippines, Brunei, Malaysia, Taiwan and Vietnam.

The claims over maritime boundaries are considered under the United Nations Convention on the Law of the Sea (UNCLOS). Vietnam has previously put its claim to the islands under UNCLOS before the United Nations, but the dispute remains an ongoing geographical issue among the countries.