China issued its highest typhoon warning on Thursday as Super Typhoon Saola drew closer to Hong Kong and the mainland's southern coast, with multiple cities delaying the start of the school year as a precaution.

Saola's wind speeds topped 200 kilometres (125 miles) per hour as of 6 pm (1000 GMT) on Thursday, with the storm around 370 kilometres southeast of the Chinese finance hub, according to the Hong Kong Observatory.

On Friday it will bring "heavy squally showers and violent winds", the observatory said, adding that the threat level was expected to be upgraded to "T8" -- the city's third-highest -- early in the morning.

On the mainland, China issued the most severe warning in its four-tiered system, with the country's National Meteorological Centre predicting that the typhoon would make landfall "somewhere in the coastal areas stretching from Huilai to Hong Kong" by Friday afternoon at the earliest.

But there was also a possibility it could move west and "pass through the waters of eastern Guangdong without making landfall".

Train services have been suspended across Guangdong, the state news agency Xinhua reported, while various cities in the southern province -- including Shantou, Shanwei, Jieyang and Chaozhou -- have pushed the start of the academic year to Monday.

Hong Kong's low-lying areas could see "serious flooding", its observatory said, and if Saola skirted the south of the territory, the storm surge "may be similar" to that seen during the 2018 Super Typhoon Mangkhut.

Mangkhut -- which triggered Hong Kong's maximum "T10" typhoon alert -- caused severe damage to the city and left more than 300 people injured.

Saola displaced thousands earlier this week as it passed the northern Philippines, but no direct casualties have been reported from it so far.

Authorities in the neighbouring casino hub of Macau said they were eyeing the possibility of issuing its third-highest typhoon warning on Saturday.

Southern China is frequently hit in summer and autumn by typhoons that form in the warm oceans east of the Philippines and then travel west.

While they can cause temporary disruption to cities like Hong Kong and Macau, fatalities have become much less common thanks to stronger building codes and better flood management systems.