The government's counter terrorism unit is stepping up a publicity campaign in hardware stores and shops that sell chemicals to try to make it harder for terrorists to buy raw materials to make bombs.

Officials from the Inter-departmental Counter Terrorism Unit (ICTU) say authorities have uncovered almost 20 explosives or precursor chemical cases since the social unrest of 2019.

They say despite the introduction of the national security law, the activities of extremists have not abated and they have now simply gone underground.

"The pattern is the same as overseas terrorist incidents, where terrorists aimed to acquire precursor chemicals from street level or outlets. That's why I think it's one of the vulnerabilities in Hong Kong," said police senior superintendent Peter Leung.

The ICTU says it hopes its new "Safe Community Pledge" will raise awareness of the terrorism threat among staff of hardware stores and logistics companies, as well as laboratories at universities and secondary schools.

Fourteen shops and almost 50 schools have signed up for the scheme so far, pledging to be alert, manage materials safely, and report anything suspicious – such as people buying an unusual amount of chemicals and paying in cash.

The campaign could later be extended to other sectors, including security, building management and hotels.