The concentration of carbon dioxide (CO2) in the atmosphere has hit a new high, the UN said on Monday, warning that drastic action is needed to achieve targets set by the Paris climate agreement.

"Concentrations of carbon dioxide in the atmosphere surged at a record-breaking speed in 2016," the World Meteorological Organisation said.

"Globally averaged concentrations of CO2 reached 403.3 parts per million in 2016, up from 400.00 ppm in 2015 because of a combination of human activities and a strong El Nino event," it said.

The Greenhouse Gas Bulletin, the UN weather agency's annual flagship report, tracks the continent of dangerous gasses in the atmosphere in the post-industrial era (since 1750). The report also said that the last time Earth experienced similar CO2 concentration rates was three to five million years ago when the sea level was up to 20 metres higher than now.