MIAMI - Last month was the hottest May in records going back to 1880, marking the warmest first five months of any year, United States scientists have said. And that may push 2015 to be the hottest year on record.

"This was the warmest May on record," said Mr Derek Arndt, chief of the monitoring branch at the National Oceanic and Atmospheric Administration's (NOAA) National Centres for Environmental Information, on Thursday.

When global air and sea surface temperatures were averaged, May was 0.87 deg C warmer than the 20th-century average, NOAA said in its monthly report. The temperatures were the highest for May in the 1880-2015 period, "surpassing the previous record set in 2014 by 0.14 Fahrenheit (0.08 deg C)", the report said.

One-quarter of the US is experiencing drought, primarily in California and nearby West Coast states, but also in parts of the north-east and New York.

The figures for May continue the upward climb in temperatures since the start of the year, when the planet's land and ocean surfaces have been 0.85 deg C above the 20th-century average.

"This was the highest for January-May in the 1880-2015 record, surpassing the previous record set in 2010 by 0.16 Fahrenheit (0.09 deg C)," NOAA said.

"I think the chances of 2015 coming in warmer than 2014 are definitely increasing," Mr Arndt said. "We don't make predictions here, but I would not be surprised if 2015 ends up as the warmest year on record."

Last year was the warmest year in data going back to 1880.

The world's oceans have played a major role in the heatwave. Every major ocean basin has set a record for heat.

El Nino is a big driver in making the earth warmer and exacerbates the impact of climate change, Mr Arndt said.

"If climate change is the escalator going up, EL Nino is like jumping up and down," he said. "So they play together."

There is an 80 per cent chance the El Nino will persist through the first three months of 2016.

AGENCE FRANCE-PRESSE, BLOOMBERG