Saudi Arabia on Thursday extended curfew restrictions on Islam's two holiest cities to 24 hours to stem the spread of coronavirus as the number of deaths from the disease rose to 21.

The announcement comes amid uncertainty over the hajj which is due to take place at the end of July, after authorities this week urged Muslims to temporarily defer preparations for the annual pilgrimage.

"Full 24-hour curfew in Makkah and Madinah starting from today until further notice," the official Saudi Press Agency reported, citing an interior ministry source.

The cities were earlier under a 15-hour daily curfew.

Authorities have already sealed off Makkah and Madinah along with Riyadh and Jeddah, barring people from entering and exiting the cities as well as prohibiting movement between all provinces.

Last month, Saudi Arabia suspended the year-round "umrah" pilgrimage over fears of the coronavirus pandemic spreading to Islam's holiest cities.

Authorities are yet to announce whether they will proceed with this year's hajj.

Last year, some 2.5 million faithful travelled to Saudi Arabia from across the world to take part in the hajj, which all Muslims must perform at least once in their lives if able.

Saudi Arabia, which has reported the highest number of infections in the Gulf, is scrambling to limit the spread of the disease at home.

On Thursday the health ministry said the deaths from the illness had risen to 21 while 1,885 infections were reported.

The Arab world's biggest economy has also closed down cinemas, malls, restaurants and halted flights as it steps up efforts to contain the virus.

King Salman has warned of a "more difficult" fight ahead against the virus, as the kingdom faces the economic double blow of virus-led shutdowns and crashing oil prices.