(CNN) China claimed Thursday that the addition of fresh military troops to the Hong Kong garrison was simply a military rotation, sparking skepticism among US officials and widespread concern in Hong Kong, demonstrating how tense the city is ahead of a 13th consecutive weekend of anti- government protests .

Chinese Defense Ministry spokesman Ren Guoqiang said the China's People's Liberation Army troop "rotation" in Hong Kong was a "routine arrangement." China has stationed troops in Hong Kong since it was turned over to the Chinese in 1997 from Great Britain.

The spokesman said the timing of this year's rotation was "similar to previous rotations" and "was decided based on the order of the Central Military Commission and the defense needs of the Hong Kong Special Administrative Region." He dodged a question about whether the number of troops and the amount of equipment have increased since the latest rotation.

The People's Liberation Army troop movements were carried out in the dead of the night and took place at the same time as a rotation of the garrison in neighboring Macao. They were announced by Chinese state media early Thursday morning.

Video: #PLA #HongKong Garrison conducted rotation of its members. The move is a normal routine annual rotation approved by Central Military Commission and in line with law. (Video via China National Radio) pic.twitter.com/JumKmGMB55

While the rotation occurred at almost exactly the same time as last year , any PLA movements are tightly scrutinized for suggestions the military could be deployed to Hong Kong's streets to tackle the ongoing unrest.

US military officials are concerned about the new movements and for now are treating the Chinese claim that they are part of a routine annual rotation with extreme skepticism, according to two US defense officials.

"This is the time to start worrying," one official said. The specific concern in the next few days is that the Chinese troop movements, including tactical vehicles and personnel carriers, puts Beijing's forces into a more immediate position to crack down on a pro-democracy rally that had been planned for Saturday, but has already been banned.

The same official added that if the troop movements were really just part of a planned rotation, China might have delayed it until after the sensitivities of Saturday.

"We are monitoring the Chinese military movements in and around Hong Kong closely," Pentagon Lt. Col. David Eastburn said in a statement. "We stand with the G7 countries in calling for violence to be avoided and in reaffirming the importance of the Sino-British Joint Declaration."

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The US is watching very carefully for what might happen next. Officials acknowledge that if China begins to control or limit communications out of Hong Kong, it could leave the Trump administration with a delayed understanding of what is happening.

On Wednesday, US Secretary of Defense Mark Esper said that "it is clear that China is engaging in -- in a deliberate strategy to undermine the stability of the region. It is clear the values and behaviors of the Chinese Communist Party do not align with the vast majority of states."

The troop movements also came as Chief Executive Carrie Lam would not rule out using emergency powers -- which would give her the right to pass new laws without the approval of legislators -- if violent protests continue.

Police denied permission for the major Saturday protest, which had been planned by the Civil Human Rights Front. The CHRF previously organized three peaceful marches that it said attracted more than a million participants. Organizers said they are appealing the police decision.

City on edge

While fears about a military crackdown have often focused on visions of PLA tanks crossing the border, the military has had a major presence in Hong Kong since the city was handed over from British to Chinese control in 1997.

Under the city's constitution , the Hong Kong government can request the assistance of the garrison "in the maintenance of public order and in disaster relief."

Administration and police officials have repeatedly denied any need to call on the military, even as the protests have taken an increasingly violent turn.

This has not stopped the rampant concern that at some point Beijing could crack down, a concern that has been boosted by a large buildup of paramilitary forces across the border in Shenzhen

While most analysts agree that the presence in the Chinese city of the People's Armed Police, a force under the military commission, is likely intended to send a message to domestic audiences that the government is in control and will not allow unrest to spill over the border, it has caused concern among many in Hong Kong.

Lam's apparent suggestion that she could invoke emergency powers has deepened that concern. Police resources have been stretched by the months-long protests, and were the government to institute a curfew or other radical action that such a move would allow, it's likely they would require reinforcement, potentially from the military.