Dozens of soldiers emerged from their barracks in Hong Kong on Saturday to help clean up nearby streets of debris and barricades strewn out by pro-democracy protesters.

The brief clean-up operation came at the end of a week of particularly intense clashes between police and protesters that had caused chaos across Hong Kong.

"There were some Hong Kong citizens clearing roadblocks near the PLA Hong Kong garrison," Chinese defence ministry spokesman Wu Qian told reporters at a security event in Bangkok when asked about the brief deployment.

"The soldiers from the PLA Hong Kong garrison joined these citizens in clearing these roadblocks and their efforts were welcomed by Hong Kong citizens."

Throughout the crisis, China has repeatedly warned that it will not give in to any of the protesters' demands, and that it will not tolerate enduring violence in the city.

After another weekend of violence and protesters setting fire to the entrance of a university entrance on Monday morning to defend themselves from police, Wu issued another warning.

"Ending violence and restoring order is the most pressing task we have in Hong Kong," Wu said after China's defence minister Wei Fenghe met his US counterpart, Mark Esper, in Bangkok.

China's People Liberations' Army has maintained a garrison in Hong Kong since the former British colony was returned to China in 1997.

But its troops rarely leave their barracks. The last time soldiers were seen on Hong Kong's streets was in 2018 to clean up after a typhoon.

According to Hong Kong's Basic Law, troops from mainland China can operate in the city if local authorities request it.

Hong Kong authorities may, "when necessary", ask the central government for assistance from the garrison "in the maintenance of public order and in disaster relief", according to Article 14.

Article 18 of the Basic Law allows the central government to effectively suspend Hong Kong's laws if there is a "state of war" or "turmoil" which "endangers national security or unity".

Hong Kong's government said on Saturday that it did not request the Chinese troops to clean up the streets.