Covid-19 is easily the most severe global health emergency ever declared by the World Health Organization (WHO), its leader has said.

Dr Tedros Adhanom Ghebreyesus said he would reconvene the WHO's emergency committee this week for a review.

There have been five other global health emergencies: Ebola (two outbreaks), Zika, polio and swine flu.

More than 16m cases of Covid-19 have been reported since January, and more than 650,000 deaths.

"When I declared a public health emergency of international concern on 30 January... there were less than 100 cases outside of China, and no deaths," Dr Tedros said.

"Covid-19 has changed our world. It has brought people, communities and nations together, and driven them apart."

The total number of cases, he added, had roughly doubled in the past six weeks.

Although the world had made a huge effort in fighting the virus, there remained "a long hard road ahead of us", he said.

In other developments:

• US President Donald Trump's national security adviser, Robert O'Brien, has tested positive for coronavirus. He is the highest-ranking official in the administration known to have tested positive
• Spain insists recent outbreaks of new cases were isolated and the country is safe for tourists to visit, after the UK introduced new measures requiring visitors from Spain to quarantine for 14 days
• Vietnam has closed the coastal city of Da Nang to tourists, after 15 new locally transmitted coronavirus cases were recorded there - the first in the country since April
• US biotech company Moderna has begun the final phase of clinical trials for a potential Covid-19 vaccine. Some 30,000 volunteers are taking part in the third and last phase of testing before the vaccine can be submitted to a regulatory authority for evaluation and possible approval
• Belgium is tightening restrictions to try to avoid another lockdown because of a worrying rise in the number of its cases. From Wednesday, Belgians will be allowed to see a maximum of five people outside of their families. Currently a Belgian individual can meet 15 people in a "social bubble"

At Monday's briefing in Geneva, Switzerland, the WHO also said travel restrictions could not be the answer for the long term, and countries had to do more to halt the spread by adopting proven strategies such as social distancing and wearing masks.

"It is going to be almost impossible for individual countries to keep their borders shut for the foreseeable future. Economies have to open up, people have to work, trade has to resume," WHO emergencies programme director Mike Ryan said.

WHO officials acknowledged however that further lockdowns in countries experiencing renewed outbreaks may be necessary, but suggested they should be as short as possible, and confined to as small a geographic area as possible (ie local lockdowns).

"The more we understand about the virus, the more surgical we can be in controlling it," said Mr Ryan.