(Bloomberg) -- The number of Chinese maritime militia vessels around key features across the South China Sea grew by 35% last year as Beijing continued to bolster its presence amid rising regional tensions, according to data by the Washington-based Asia Maritime Transparency Initiative.

A report released on Wednesday and based on satellite imagery of nine features in the sea, including reefs and shoals, identified an average of 195 militia ships present on any given day.

The increase from a year earlier was accompanied by “a dramatic shift” in vessels to Mischief Reef in the summer of 2023 when over 180 boats were observed gathered there from July. The reef is also claimed by the Philippines and Vietnam.

“The data shows that China’s militia is as active as ever,” the report says. “The reason for this increase, an anomaly compared to the previous year’s peak of only 37 vessels at Mischief, is unclear.”

The initiative, part of the Center for Strategic and International Studies, describes China’s maritime militia as “a force of vessels ostensibly engaged in commercial fishing but which in fact operate alongside Chinese law enforcement and military to achieve political objectives in disputed waters.”

Located within the Philippines’ exclusive economic zone, Mischief Reef has been controlled by China for nearly three decades, with Manila accusing Beijing of turning it into a “militarized artificial island.” In response, Manila has established a military outpost at the nearby Second Thomas Shoal in the form of a rusting warship it grounded there in 1999.

Last year, that ship — the BRP Sierra Madre — became the center of renewed tensions between the two nations amid water cannon fire and near collisions with Chinese ships during attempts to resupply troops stationed at the outpost.

Read More: Race to Fix Crumbling Ship Threatens Conflict in South China Sea

While Chinese militia vessels seen active around the Second Thomas Shoal typically operate out of Mischief, the summer surge at the reef appears mostly unrelated to efforts to oppose the resupply missions, AMTI said.

Instead, the purpose-built professional militia ships from the southern Chinese province of Hainan “routinely worked with the China Coast Guard to physically oppose Philippine resupply missions to the BRP Sierra Madre.”

Apart from the groupings at Mischief Reef, the largest consistent collection of militia ships continued to be seen at Hughes and Whitsun reefs, the report said. A persistent militia presence was also maintained near China’s outpost at Gaven Reefs, and smaller groups could be seen at the reefs east of Philippine occupied Thitu and at Iroquois Reef.

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