The semi-autonomous city lacks its own refineries and imports its entire supply of automotive fuel, with much of it coming from neighbouring mainland China.

Prices at Hong Kong's filling stations are the world's most expensive, creating an incentive for smugglers to transport cheaper fuel illegally across the border into the city.

Hong Kong fire service officials said they had observed a "sharp rise in... complaints" related to illicit fuelling activities, receiving 154 of them in March, about 40 percent more than the average over the previous two months.

Customs authorities seized nearly 19,500 litres of illicit fuel last month, more than the total amount confiscated in January and February.

Makeshift petrol stations with inadequate facilities are "posing intolerable risks to public safety", fire officials said.

Smugglers were increasingly modifying seven-seater cars and light goods vehicles, and disguising them for use as mobile fuelling stations, they said.

Authorities said they were using robots and X-ray scanners at the border with mainland China to deter smuggling and had deployed aerial drones for surveillance.

The retail price of standard petrol reached about HK$32.40 (US$4.14) per litre in Hong Kong this week, according to the city's Consumer Council.

Authorities launched temporary measures on Thursday, including a subsidy for diesel-powered public transport, to cushion the impact of soaring fuel costs.