Governments are "not on track" to achieve a target of keeping the average global temperature rise below two degrees Celsius (3.6 degrees Fahrenheit), the UN climate chief said Thursday.

Christiana Figueres, executive secretary of the United Nations Framework Convention on Climate Change (UNFCCC), told a forum in Singapore that the world was moving in the right direction, but not fast enough.

"Even if governments were to comply with all the mitigation pledges that are on the table, it will still only provide 60 percent of the effort that is necessary to keep global average temperature rise to under two degrees," she said.

"It is also well known that governments have agreed to review this target to 1.5 degrees should the science demand more drastic action. Frankly... we are not on track."

World leaders agreed in December 2009 to the Copenhagen Accord, which introduced a plan to cap the rise in temperatures to below two degrees to stave off the worst effects of climate change.

"We are clearly moving toward a low-carbon economy. What we're not doing is we're not moving with the speed and at the scale that the science demands," Figueres said.

Her predecessor Yvo de Boer said in March that the target was already out of reach. It was set by a core group of countries in the final stormy hours at the Copenhagen Summit and became enshrined at Cancun, Mexico a year later.

More and more scientists are warning that the objective is slipping away without radical, early cuts in greenhouse-gas emissions.