A cyclone more powerful than any previously recorded in southern Oman has slammed into the Gulf country and neighbouring Yemen, deluging a major city with nearly three years' worth of rainfall in a single day and leaving 13 people dead.

Key points: Eight people are still missing

Eight people are still missing Cyclone Mekunu brings wind gusts of 200kph

Cyclone Mekunu brings wind gusts of 200kph Key Port of Salalah remains closed

Eight people were also missing as Cyclone Mekunu caused flash flooding that tore away whole roadways and submerged others in Salalah, Oman's third-largest city, stranding drivers.

The dead include a 12-year-old girl killed when the storm's strong winds flung open a metal door that struck her in the head.

Rushing waters from the rain and storm surges flooded typically dry creek beds.

The holiday destination's now-empty tourist beaches were littered with debris and foam from the churning Arabian Sea.

India's Meteorological Department said the storm packed maximum sustained winds of 170 to 180 kilometres per hour with gusts of up to 200kph.

It called the cyclone "extremely severe".

The floods tore apart whole roadways. ( AP: Kamran Jebreili )

Branches and leaves littered the streets. Several underpasses became standing lakes and cars were left abandoned on the road.

The Port of Salalah — a key gateway for the country and for Qatar amid a regional diplomatic dispute — remained closed, its cranes secured against the pounding rain and winds.

Salalah Port Services said it would take a minimum of 72 hours to "make the port safe".

Omani forecasters said Salalah and the surrounding area would get at least 200 millimetres of rain, over twice the city's annual downfall.

It actually received 278.2mm, nearly three times its annual rainfall.

The country's Labour Ministry declared a three-day holiday for companies and establishments in the private sector in the Dhofar region, state news agency ONA reported.

The central bank, meanwhile, issued a circular announcing a three-day holiday starting on Sunday for money exchange houses, banks, finance and leasing companies in Dhofar.

Before hitting Oman, Mekunu struck Yemen's Socotra in the Arabian Sea, causing massive damage to the island that UNESCO has recognized as a world natural heritage site.

Over 500 families were displaced by the storm.

While Salalah International Airport has already reopened, there is still much damage that needs to be repaired in Oman, from downed power lines to whole roadways swallowed by raging flash floods. Water supplies also have been affected.

Several underpasses in Oman have turned into waterways. ( AP: Kamran Jebreili )

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