The South Korean flag was raised for the first time at a Nato cyber defence centre in Tallinn, the Estonian capital, on Thursday, when Seoul became the first in Asia to join the group.
Nato's Cooperative Cyber Defence Centre of Excellence trains specialists from member states to work together to fend off cyberattacks and South Korea is the fifth non-Nato member to sign up for it.
South Korea's National Intelligence Service said it had been trying to join the cyber defence centre since 2019 to learn more about threat response strategies and ways to protect key infrastructure, with the broad aim of having world-class abilities to respond to those dangers.
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Although the centre is separate from the Nato command structure, Chinese military analysts said that the addition of its close neighbour and American ally to the group had Beijing worried, seeing it as expansion of the US-led defence alliance in northeast Asia that could threaten Chinese security interests in the region.
China has repeatedly said it is opposed to the enlargement of Nato.
Russia has used the military alliance's eastward expansion to justify its invasion of Ukraine, and Beijing has called for Western countries to consider what it says are Moscow's legitimate security concerns.
Shanghai-based military analyst Ni Lexiong said China saw Nato as being overbearing and expanding, and South Korea's decision to join the centre "definitely does not benefit China".
But Seoul would have considered Beijing's interests and its friendship with China, he said.
Ni said South Korea was a small country surrounded by military giants with conflicting interests and would not actively damage those ties and put its security position in peril.
However, it also faced threats from North Korea that required it to strengthen its defensive capabilities.
"It also needs China to influence and exert pressure on North Korea to restrain the latter's actions," he said.
Ni said China could also be worried about Nato training for South Korea because China would have to show support to North Korea if a conflict erupted on the Korean peninsula.
South Korean president-elect Yoon Suk-yeol, who will take office on Tuesday, has said he will take a harder line on Pyongyang. He is expected to abandon his predecessor's Sunshine Policy of seeking more amicable ties with North Korea.