The number of coronavirus cases recorded worldwide topped 18 million on Monday with the illness continuing its march, after Australia's second-largest city imposed a curfew to halt the spread.

Six months after the World Health Organization declared a global emergency, the virus has killed more than 687,000 people since it first emerged in China late last year, according to an AFP tally compiled from official sources.

Fresh clusters have been reported in countries that had previously brought their outbreaks under control, forcing governments to reimpose lockdown measures despite worries over further economic fallout.

Australia's Victoria state imposed fresh, sweeping restrictions on Sunday, including a curfew in Melbourne for the next six weeks, a ban on wedding gatherings, and an order that schools and universities go back online in the coming days.

"Anything short of this will see it drag on for months and months and months," Victoria premier Daniel Andrews said of the outbreak.

Despite a lockdown, Melbourne has continued to report hundreds of new cases daily even as other states in Australia have reported zero or a small number. Many other parts of the world are struggling with much bigger outbreaks.

Health authorities in South Africa, where a surge in cases had been expected after the gradual loosening of a strict lockdown, reported that infections exceeded the half-million mark.

The nation is by far the hardest-hit in Africa, accounting for more than half of diagnosed infections, although President Cyril Ramaphosa said the fatality rate is lower than the global average. Latin America and the Caribbean passed another milestone on Sunday as fatalities in the region climbed to more than 200,000, with Brazil and Mexico accounting for nearly three-quarters.

Iran -- battling the Middle East's deadliest outbreak -- reported its highest single-day infection count in nearly a month, warning that most of its provinces have been hit by a resurgence of the disease.

With infections and deaths soaring, the UN health agency has said the coronavirus pandemic is likely to be protracted and warned of possible "response fatigue".

"The WHO continues to assess the global risk level of COVID-19 to be very high," the agency said, adding that the effects of the pandemic "will be felt for decades to come".

Vaccine race

Mexico overtook Britain to become the third hardest-hit country in virus deaths -- after Brazil and the United States -- with more than 46,600 fatal cases. The US has now tallied more than 4.6 million cases and 154,793 deaths.

The pandemic has spurred a race for a vaccine with several Chinese companies at the forefront, while Russia has set a target date of September to roll out its own prophylactic. However, US infectious disease expert Anthony Fauci said it was unlikely America would use any vaccine developed in either nation.

As part of its "Operation Warp Speed", the US government will pay pharmaceutical giants Sanofi and GSK up to $2.1 billion for the development of a COVID-19 vaccine, the two companies have said.

In Europe, where many countries had hoped their outbreaks had been brought under control, Norway recorded its first virus death in two weeks and Switzerland reported its case numbers had crept up again. Despite the resurgence, Europe has seen demonstrations against coronavirus curbs.

Thousands protested in Berlin over the weekend urging "a day of freedom" from the restrictions, with some demonstrators dubbing the pandemic "the biggest conspiracy theory". The protests, in which many demonstrators failed to wear masks or respect social distancing rules, triggered calls for tougher penalties against those who violate curbs.

At least 45 police officers were injured and more than 130 people were arrested. The pandemic has also continued to cause mayhem in the travel, sports, cultural and tourism sectors, with more airlines announcing mass job cuts and major festivals and cultural events scaling back.

Latin America's biggest airline, the Brazilian-Chilean group LATAM, said it would lay off at least 2,700 crew, and British Airways pilots overwhelmingly voted to accept a deal cutting wages by 20 percent, with 270 jobs lost.

Austria's month-long Salzburg festival celebrates its 100th anniversary, but now with a reduced programme and strict safety measures, including masks for spectators until they are seated. The 80,000 tickets for the event -- down from the usual 230,000 -- have been personalised to enable contact-tracing.