Chinese research vessels with ties to the People’s Liberation Army (PLA) are conducting sweeping surveys of the undersea floor in the Indian Ocean, collecting data that could be crucial in deploying submarines in a region that is a critical energy supply line for Beijing in the event of a war with Taiwan.

A new analysis of hundreds of thousands of hours of shipping data since 2020 by the Washington-based Center for Strategic and International Studies shows that the Indian Ocean is fast becoming one of the biggest domains for Chinese oceanic surveys, which are ostensibly civilian in nature but tied to the PLA and Beijing’s military-civil fusion program — a national strategic plan to advance China’s military by acquiring technology and research from civilian groups.

The types of ocean surveys carried out by the vessels have research applications for energy resources and marine environments, but the data collected can also be used for military purposes, analysts say, including how to maneuver and obscure submarines during conflict.

The CSIS report found that of the 13 vessels undertaking the bulk of survey and research activity in the Indian Ocean since 2020, all have links to China’s military — including organizational ties to the PLA — and have displayed suspicious behavior including docking at Chinese military ports or temporarily turning off tracking devices.

“The Indian Ocean is critical to China’s strategic and economic interests, as well as its geopolitical rivalry with India,” said Matthew Funaiole, a senior fellow at CSIS who worked on the report. “Beijing is serious about fielding a blue-water navy, one that will be active in the Indian Ocean, and blurring the lines between its research ecosystem and its national security apparatus will help it get there.”

China maintains the world’s largest fleet of civilian research vessels, and the CSIS report said that at least 80 percent of 64 such vessels operating globally since 2020 have displayed “warning indicators” that their work is tied to military objectives. Over half of those suspect vessels operated in the South China Sea, but their growing presence in the Indian Ocean has also stoked tensions.

Last week, Sri Lanka declared a moratorium on Chinese research vessels entering its waters under what analysts say was intense pressure from India. New Delhi has aired concerns that the research vessels — some of which have previously docked in Sri Lanka — are being used to monitor waters and installments in India’s sphere of influence. Sri Lanka, which took on nearly $12 billion in Chinese loans between 2000 and 2020, has struggled to balance the competing demands of Beijing and New Delhi, analysts say.

“India has made its displeasure known to Sri Lanka; some of these vessels are too close to Indian territory and Indian interests for comfort,” said Abhijit Singh, a former Indian naval officer and senior fellow at the New Delhi-based think tank Observer Research Foundation. “Crudely put, this is the real fear, that China is going to work on its combat capability by studying the environment in these waters.”

The Chinese Embassy in India did not respond to a request for comment.

With the backdrop of China’s growing military presence, the Biden administration has sought to tighten security ties with India in recent years, including bolstering activity in the Quad — a group including India, Japan, Australia and the United States that focuses on security and economic interests in the Indo-Pacific region.

More recently, the White House has sought to show its ties with New Delhi remain strong, despite a spat over the alleged attempted assassination of a Sikh separatist by an Indian government employee on U.S. soil. Last month, deputy U.S. national security adviser Jon Finer led a delegation to India to fortify partnerships in technology.

Much of the attention on China’s growing military presence in the Indo-Pacific has focused on its massive fleet of naval ships and increasingly assertive aircraft maneuvers near Taiwan. However, beneath the oceans, Beijing is also working to expand a less-visible network of submarine defense systems and ocean monitoring equipment that would be critical in supporting its naval defenses and protecting supply routes in the event of war.

The Indian Ocean is a critical waterway for Beijing’s interests, and in recent years it has built or expanded facilities from Djibouti to Pakistan. While China has made efforts to supplement its ocean supply line with overland alternatives in recent years, a substantial amount of its crude oil and natural gas supplies still need to travel from Africa and the Middle East through the Indian Ocean, including the chokepoint of the Malacca Strait, which connects the Indian Ocean and the South China Sea between Indonesia and Malaysia.

“Some call that the underbelly of China’s strategic interests, simply because if a war in Taiwan erupts, then given that the Indian Ocean is located quite a distance from the Chinese shores it’s easy to disrupt the Chinese energy security supply in the Indian Ocean, and then all war-making ability might grind to a stop,” said Collin Koh, a senior fellow at the S. Rajaratnam School of International Studies in Singapore.

China’s fleet of submarines is growing fast. Last year, in an annual report on China’s military power, the Pentagon said Beijing now has about 60 submarines, including 12 that are nuclear-powered, and projects that the total number of Chinese submarines will rise to 80 by 2035. Some of these ships have already made forays into the Indian Ocean.

Last year, the United States, Britain and Australia unveiled plans to equip Canberra with its own nuclear-powered submarines as part of a landmark agreement called AUKUS, designed to counter China’s growing presence in the region.

“If you are serious about wanting to conduct submarine operations in the Indian Ocean you have to have a fairly good knowledge of not only the seafloor, but the currents, the layers of water, the salinity … which is all key to not being seen when you’re in a submarine,” said David Brewster, senior research fellow with the National Security College at the Australian National University.

China is not alone in deploying ocean research vessels, but the opaque ties among its military, civilian and academic groups have raised suspicions that the data collected in the Indian Ocean and elsewhere globally could have dual-use applications. In some cases, the link between the missions and China’s national security goals is made explicit.

In 2020, Chinese research survey vessel Xiang Yang Hong 06 traveled more than 6,000 miles over 110 days, surveying vast swaths of the Indian Ocean. During that time it deployed underwater gliders and floats — devices to capture complex data about the marine environment — as part of a national project called “Two Oceans One Sea,” which, according to descriptions posted by Chinese state research groups, is designed to advance strategic needs including “security and military activities.”

In October 2023, another research vessel, Shiyan 06, conducted a four-month mission in the eastern Indian Ocean. The vessel was operated by the South China Sea Institute of Oceanology, an institute that has provided technical support to Beijing’s military expansion in the South China Sea.

Analysts say the behavior of the ships in the Indian Ocean and elsewhere gives insight into their affiliation with military groups. “Certain indicators can fill gaps in our knowledge. If a research vessel is owned and operated by a state-affiliated group with close ties to the Chinese military, and that vessel makes regular port calls at naval facilities, it’s a red flag,” said CSIS’s Funaiole. “If a vessel regularly goes dark before entering another country’s exclusive economic zone, it’s another red flag.”

The Pentagon has taken note of the growing PLA navy presence in the Indian Ocean, including its expanded submarine activities. “The PLAN has also conducted submarine deployments to the Indian Ocean, demonstrating its increasing familiarity in that region and underscoring the [People’s Republic of China’s] interest in protecting [sea lines of communication] beyond the South China Sea,” it said in its 2023 military report on China.