The observatory said it will issue the Increasing Gale or Storm Signal No. 9 at 1.40am on Wednesday with Super Typhoon Ragasa expected to bring hurricane-force winds to Hong Kong as it continues to get closer.

The forecaster said parts of the city are affected by gale force winds.

"Super Typhoon Ragasa is expected to be closest to Hong Kong [on Wednesday morning], skirting around 100 kilometres to the south of the territory. Its hurricane force winds will edge close to Hong Kong gradually," it said.

As of midnight, the storm was centred about 190 kilometres southeast of the territory, and is forecast to move west-northwest at about 22 kilometres per hour towards the western Guangdong coast.

"Ragasa has an extensive circulation with fierce winds. The local weather will be persistently adverse [on Wednesday] with frequent heavy squally showers and thunderstorms.

"Seas will be phenomenal with swells and overtopping waves over the shoreline, which will be particularly significant along the eastern and southern coasts."

Gusts of wind in excess of 100 km per hour have been recorded at Tate’s Cairn, Waglan Island, Green Island, Cheung Chau and by the Hong Kong Sea School.

The MTR said it will maintain its train services at limited frequencies until it shuts down for the night.

However, if a T9 signal or higher is still in effect by the time it resumes operations in the morning, it said train services running along open-air sections – including light rail services – will be suspended.

The No. 8 signal has been in effect since 2.20pm on Tuesday.

All commercial flights have been halted in and out of Hong Kong as Ragasa approaches.

The Airport Authority said it handled around 600 scheduled flights on Tuesday, and "all passengers in the restricted areas of the terminal building have all departed by air or left".

The airport's website showed the last flight that departed took off at 5.59pm.

"Adverse weather is expected to continue affecting flight operations, with only a limited number of cargo flights scheduled for [Wednesday]," the authority said.

Meanwhile, the Hospital Authority said four people – two males and two females – had sought treatment at the accident and emergency units at public hospitals as of 10.50pm.

As of 11pm, authorities had received 12 reports of fallen trees and one report of a landslide.