MIAMI (AFP) - Last month marked the hottest May in modern history, continuing a troubling trend of rising global temperatures, US government scientists said Thursday.

"This was the warmest May on record," said Derek Arndt, chief of the monitoring branch at the National Oceanic and Atmospheric Administration (NOAA) National Centres for Environmental Information, during a conference call with reporters.

When global air and sea surface temperatures were averaged, May was 0.87 deg C warmer than the 20th century average, said the NOAA monthly report.

The temperatures were the highest for May in the 1880-2015 period, "surpassing the previous record set last year in 2014 by 0.14 Fahrenheit (0.08 deg C)," said the NOAA report.

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

The monthly figures for May continue an upward climb for 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.

Last year was the hottest on record, the US agency has said.