Chief Executive John Lee on Tuesday warned that the spying conviction of a Hong Kong resident by a mainland court showed there are still national security risks lurking within the community.

He was speaking a day after the Intermediate People's Court in Suzhou sentenced John Leung, 78, to life imprisonment for espionage. The court also stripped the US passport-holder of his political rights and fined him 500,000 yuan.

Lee said the case demonstrated the need to stay vigilant against national security risks.

"This incident tells us that national security risks still lurk within the community," he said.

"We have stressed repeatedly that even though Hong Kong is now generally stable, we must not let our guard down against any threats to national security. The black riots and the Hong Kong version of the colour revolution of 2019 should serve as a warning for us – that we always have to be vigilant in the long run against hidden threats, including soft resistance."

Asked whether the authorities had given Leung any assistance, Lee said there's a mechanism in place for Hong Kong authorities to be alerted of any arrests made across the border.

"The main purpose of this reporting mechanism is to allow, first of all, the family members to know about the situation, so that they can do what is necessary to help the person concerned. Equally, if there is any request for assistance to the Hong Kong government, we will offer assistance as much as we can as requested," he said in his regular press appearance ahead of the weekly Executive Council meeting.

The CE also defended the pulling of various books from public libraries, after recent media reports found that works by a number of authors had been removed from library shelves, including non-political titles.

Lee said these books could still be found in private-sector bookshops, and stressed that public libraries have a duty to manage their titles.

"The principles that we use, which I support, are to ensure that there is no breach of any laws in Hong Kong, including copyrights et cetera, and also, if they spread any kind of messages that are not in the interest of Hong Kong," he said.

"That is an important thing I think any government needs to do, so that the books we allow for public circulation do serve the interest of Hong Kong. I have strong confidence in the professionalism of the colleagues involved."