2020 was one of the three hottest years on record, marked by wildfires, droughts, floods and melting glaciers, a United Nations report said on Monday, prompting the UN Secretary-General to say the world stands "on the verge of the abyss".

In a "double blow" to millions hit by the extreme climate events, lockdown restrictions linked to the global coronavirus pandemic also delayed crucial assistance in some regions, said the report by the World Meteorological Organization (WMO).

The 'State of the Global Climate' report confirms its preliminary findings, including on land and ocean temperatures, and comes ahead of a U.S.-led summit on April 22-23 where Washington will seek to restore its credibility on climate change through fresh pledges.

"We are on the verge of the abyss... We are seeing record levels in tropical storms, in the melting of ice sheets or glaciers, in relation to drought, heat waves and wildfires," UN Secretary-General Antonio Guterres told Reuters.

The report showed the global average temperature in 2020 was about 1.2 degrees Celsius above the pre-industrial period, ranking it somewhere in the top three hottest years alongside 2016 and 2019 despite cooling La Niña conditions.

A WMO spokeswoman said it was too close to make a call between them because of the margin of statistical error.

Among the indicators highlighted were record low Arctic sea ice extents in two months of 2020. Around 80 percent of the ocean experienced at least one marine heatwave last year.

The United States saw both the largest fires ever and the most hurricanes making landfall. Hurricane Laura alone, which hit Louisiana on Aug. 27, caused $19 billion in economic losses.

Death Valley in California also recorded the world's highest known temperature for at least 80 years of 54.4 degrees Celsius (129.92°F) in August, the report said.

Measures designed to curb the spread of Covid-19 delayed assistance after Cyclone Harold, one of the strongest storms ever seen in the South Pacific, and after Cyclone Vongfong in the Philippines, where social distancing rules meant residents could not be evacuated in large numbers.