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Hong Kong residents woke up Thursday morning to images and videos of armored personnel carriers, army trucks, patrol boats, and armed helicopters that had crossed the border from China into the city-state under the cover of night.

But Beijing said there was nothing to worry about.

The military maneuvers were part of what the Chinese defense ministry said were “routine annual rotations,” but the timing of the action — after three months of increasingly tense and violent pro-democracy protests — has heightened nerves in Hong Kong even further.

Since the U.K. handed Hong Kong back to China in 1997, China typically has somewhere between 6,000 and 10,000 troops in Hong Kong at any given moment. The troops are rarely seen in Hong Kong, and are there to be called upon by the Hong Kong government only in an emergency.

Footage of the troops’ movements was aired on state-run television station CCTV, showing armored carriers, trucks, and a patrol boat crossing the border by night from the neighboring city of Shenzhen. Dozens of soldiers were filmed running in unison onto trucks.

Later on Thursday morning, a squadron of armed helicopters from the garrison’s air force flew from a southern China army base to Shek Kong Airfield in Hong Kong.

“This time the task has a glorious mission,” a major said to troops before they departed. “The responsibility is great. The job is difficult. The time for a true test has arrived!”

The official Xinhua News Agency reported that this was the 22nd rotation of the People’s Liberation Army’s garrison in Hong Kong. The previous one was in August 2018.

However, after each of the last two years’ rotations, the PLA stated that the number of troops would remain the same. There was no such statement this year, sparking speculation that China may be planning to increase its military presence on Hong Kong.

Xinhua also reported that the new troops had undergone training “to master military skills and knowledge about the general situation in Hong Kong and relevant laws.”

Experts said the unusually early report from the Xinhua News Agency, which was published at 4 a.m. local time, was designed to allay fears of Chinese military intervention and prevent any further instability in Hong Kong.

“This rotation was carefully arranged with comprehensive consideration. If the garrison troops entered Hong Kong in the darkness secretly, without a report, that would scare the Hong Kong media and Hong Kong people,” Beijing-based military expert Zhou Chenming told the South China Morning Post. “Beijing still values the stability of Hong Kong.”

Hong Kong is a city on the edge after 12 consecutive weeks of protests that were sparked by a now-shelved extradition bill that would have allowed criminal suspects to be transferred to mainland China.

At first peaceful, the protests have turned increasingly violent in recent weeks, raising the prospect that China will seek to intervene militarily.

Hong Kong’s government can call on the PLA to bolster its own police force but have so far said they do not intend to do so. China has labeled protesters as “terrorists” and said it stands ready to help out if the Hong Kong government requests help.

This weekend is shaping up to be another one for large protests: the Civil Human Rights Front, whose previous demonstrations have attracted hundreds of thousands of people, is planning a mass march through the city on Saturday. Hong Kong’s police have however banned the march, citing intelligence that it will be used to launch further violent attacks.

The group says it plans to appeal the decision, and on social media, protesters have made it clear they’re planning to march no matter what.