Tens of thousands of people were being evacuated in the Philippines and Taiwan on Sunday as the season's first super typhoon bore down on the region with destructive winds and torrential rainfall.

Forecasters said Ragasa, known locally as Typhoon Nando, had undergone "explosive strengthening" over the Philippine Sea and was threatening to become one of the strongest storms to hit Asia this year.

At around 11am local time, the Philippine weather agency Pagasa reported maximum sustained winds of 185kmph and gusts of up to 230kmph, warning of "life-threatening flooding, landslides and storm surges" across northern Luzon.

Interior secretary Jonvic Remulla said officials "must waste no time in moving families out of danger zones", adding that pre-emptive evacuations had begun in flood-prone communities in Cagayan, Isabela, Ilocos and Batanes provinces.

In Taiwan, the Central Weather Administration said nearly 300 residents had been moved from at-risk areas in Hualien county.

"A land typhoon warning will be issued tonight, and tomorrow morning the typhoon will approach Taiwan's offshore," the agency said.

Philippine forecasters said Ragasa's impact would be felt as early as Sunday evening, with the most severe conditions expected on Monday morning.

"We expect severe flooding and landslides in northern Luzon," weather specialist John Grender Almario told a press briefing. "The strongest effects will be at 8am tomorrow."

Storm surges exceeding three metres are forecast in Batanes, northern Cagayan and Ilocos, while waves up to 11 metres will make conditions perilous at sea.

Beyond the Philippines and Taiwan, Ragasa is expected to continue westwards through the Luzon Strait before edging towards southern China.

Meteorologists warn that the system could bring storm surges similar to those caused by Typhoon Mangkhut, which inundated parts of Hong Kong in 2018.

The Hong Kong Observatory said the weather would "deteriorate gradually" on Tuesday and Wednesday, with gale-force winds and flooding likely.

With the storm expected to affect air travel, airlines such as Cathay Pacific and Hong Kong Airlines waived rebooking fees for flights between Tuesday and Thursday.

The storm comes amid widespread protests in the Philippine capital Manila over a corruption scandal linked to billions of pesos in "ghost" flood-control projects. The capital, however, is expected to be largely spared the worst of the storm.

The Philippines, which is struck by around 20 cyclones a year, is highly vulnerable to the climate crisis.

Scientists warn storms are becoming more powerful as oceans are warming.

Forecasters said Ragasa was moving northwest at 15kmh over very warm waters, an environment primed to fuel further intensification.

Ragasa isn't the only storm intensifying in the pacific ocean this week. Super Typhoon Neoguri has also reached category 4 equivalent strength. The early Sunday morning visible satellite images revealed that both typhoons were very large and undergoing rapid intensification.

Mapped: Super Typhoon Ragasa forces evacuations in Philippines and Taiwan