Washington (AFP) - The deep ocean may not be hiding heat after all, raising new questions about why global warming appears to have slowed in recent years, said the US space agency Monday.

Scientists have noticed that while greenhouse gases have continued to mount in the first part of the 21st century, global average surface air temperatures have stopped rising along with them, said NASA.

Some studies have suggested that heat is being absorbed temporarily by the deep seas, and that this so-called global warming hiatus is a temporary trend.

But latest data from satellite and direct ocean temperature measurements from 2005 to 2013 "found the ocean abyss below 1.24 miles (1,995 meters) has not warmed measurably," NASA said in a statement.

The findings present a new puzzle to scientists, but co-author Josh Willis of NASA's Jet Propulsion Laboratory (JPL) said the reality of climate change is not being thrown into doubt.

"The sea level is still rising," said Willis.

"We're just trying to understand the nitty-gritty details."

A separate study in August in the journal Science said the apparent slowdown in the Earth's surface warming in the last 15 years could be due to that heat being trapped in the deep Atlantic and Southern Ocean.

But the NASA researchers said their approach, described in the journal Nature Climate Change, is the first to test the idea using satellite observations, as well as direct temperature measurements of the upper ocean.

"The deep parts of the ocean are harder to measure," said researcher William Llovel of NASA JPL.

"The combination of satellite and direct temperature data gives us a glimpse of how much sea level rise is due to deep warming. The answer is -- not much."