A group of swimmers braved the sting of extremely salty waters to cross the Dead Sea from Jordan to Israel on Tuesday, a seven-hour challenge that organisers described as a first.

The swim organised by the EcoPeace charity was aimed at raising awareness for the iconic water body which has been receding by roughly a metre each year.

The 26 extreme swimmers from across the world, equipped with special masks due to the water's high salinity, took seven hours to make the crossing, organisers said.

The distance is only 17 kilometres (10.6 miles) and the mineral-rich lake was calm, but the high amount of salt in the water makes it nearly impossible to swim normally.

It has 10 times more salinity than the Mediterranean Sea, meaning bodies float to the top, and keeping under the water is difficult.

"It was tougher than we expected," said Samuel Moran, a 40-year-old Spaniard.

"The worst was the sun and the feeling of the salt on your skin that is very irritating. You feel like you are burning all the time," he said, adding he felt like quitting "many times".

The swimmers, the oldest of whom was 68, wore specially designed face masks as even a tiny amount of water in the mouth or eyes can cause agony.

Kim Chambers from New Zealand said she had swam extreme routes across the globe but this presented a unique test.

"Even just a few drops (of water) feels like acid burning in your eyes -- if you ingest it, either through your mouth or through your nose, it is potentially fatal."

The Dead Sea's coastline is shared by Israel, Jordan and the Palestinian territories and is the lowest point on earth, 400 metres below sea level.

But experts have warned it is on course to dry out by 2050.

Gidon Bromberg, EcoPeace Middle East co-director, said the event was "a global call to save this amazing sea".

"This is the lowest place on earth, these are the deepest saline waters on the globe, a unique composition, and sadly for the last 50 years they have been dramatically on the decline," he said.

The swimmers came from across the globe, including New Zealand, South Africa and Israel. There were no Jordanians among them.

The Israeli army did not immediately respond to a request for confirmation it was the first ever swim across the sea.

- 'Potentially fatal' -

The Dead Sea's degradation started in the 1960s when Israel, Jordan and Syria began to divert water from the Jordan River, its main source, largely for irrigation.

The water level has dropped around 40 metres since its peak, said Yechieli Yoseph, a hydrogeologist with the Geological Survey of Israel.

Companies also take out water to use the minerals inside it for medical purposes, Yoseph added.

More than 4,000 sinkholes have also formed along the Israeli, Palestinian and Jordanian coasts since the 1980s, according to Israeli research, with more than 400 per year in recent years.

"The Dead Sea is going down mostly because we have changed its water balance -- we meaning Israel, Jordan and Syria," said Yoseph.

"Of the water that used to come to the Dead Sea 90 percent of it or more is now being taken."

Standing on the shore after completing the swim, a clearly emotional Chambers said the swimmers had used the "power of swimming to show the world" the Dead Sea was in danger.

But Yoseph said the political climate between the countries involved made meaningful solutions difficult.

Israel and Jordan signed a peace treaty in 1994 but relations between the two sides remain frosty, while Syria has descended into civil war.

Yoseph said Israel could potentially afford to stop taking water as it has advanced desalination methods but it was "not a possible decision" for water-sparse Jordan.

"There is no real solution. You can talk about restraining maybe but that is also very difficult."