The probability of a volcanic eruption remains high in Iceland, the Icelandic Meteorological Office (IMO) said on Tuesday.
The eruption could take place on the Reykjanes Peninsula, it said.
National radio RUV reported that if volcanic magma were to occur offshore south of the peninsula, it could lead to the development of ash clouds, affecting air travel.
Residents who had been permitted to visit the evacuated town of Grindavik were ordered to leave quickly as levels of the chemical sulphur dioxide (SO2) in the air rose, indicating that magma had moved closer to the surface of the Earth.
Benedikt Ofeigsson, an IMO geophysicist, told RUV that magma had been detected 500 meters from the surface on Tuesday afternoon.
"SO2 is not released from magma until very close to the surface," Ofeigsson explained.
However, RUV reported that there is only a slim possibility that an eruption will take place under the sea at the very southern end of the magma intrusion.
Such an eruption would affect flights to and from Keflavik, the airport serving the Icelandic capital.
Nevertheless, RUV underlined that this would be nowhere near as serious as the 2010 eruption of the Eyjafjallajokull volcano, which occurred beneath glaciers.