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Tokyo (AFP)

Athletes voiced concerns over water quality and temperature at a marathon swimming test event for Tokyo 2020 Sunday, as officials vowed to monitor the situation closely in the run-up to the games.

"That was the warmest race I've ever done," said three-time Olympic medallist Oussama Mellouli from Tunisia after completing the 5km men's competition.

"It felt good for the first 2km then I got super overheated," added the 35-year-old, who won gold in the 10km swim at the London Olympics in 2012.

The event started at 7am with the air temperature already over 30 degrees as the Japanese capital swelters through a deadly heatwave.

"The water temperature was high so I'm a bit concerned about that," said Yumi Kida from Japan, who said she guzzled iced water before the race in an effort to reduce her body heat.

International Swimming Federation (FINA) rules state that athletes may not race when the water temperature exceeds 31 degrees and FINA's executive director Cornel Marculescu said competitors' wellbeing was top priority.

Marculescu said an external body would be set up in conjunction with Tokyo 2020 organisers to monitor both water quality and temperature in the run-up to the games and the results could affect the timing of the marathon swimming event.

"Based on this information, we will decide the time the event will start. Could be 5am, could be 5:30am, can be 6am, can be 6:30am -- depends on the water temperature," he told reporters.

"Working with a specialised company like we are going to do here in Tokyo, we will have the right information to take the right decision."

Hot weather issues have become the biggest headache for Tokyo organisers, who have already moved up the start time of several events including the marathon in a bid to mitigate the effects of the blistering heat of the Japanese summer.

- 'A little stinky' -

In terms of water quality, David Gerrard from FINA's medical committee said readings from the test event would not be ready for 48 hours but previous results gave cause for optimism.

"What we have had are readings fom the last month, daily readings that have given us very clear indications of the water quality, which has been good," he said.

Organisers are desperate to avoid the embarrassment of the Rio Olympics in 2016 when the pool used for diving events turned an unsettling shade of green overnight.

Brazilian officials also had to scramble to clean up the bay used for sailing and windsurfing that was plagued by sewer bacteria and filthy with rubbish.

In October 2017, Tokyo 2020 organisers were left red-faced after tests revealed levels of e-coli bacteria more than 20 times higher than international standards, sparking doubts about the venue's safety.

At the time, the organising committee blamed prolonged summer rain that had brought pollutants from offshore for the high readings between late July and early September.

A year later, organisers said that tests using underwater "screens" to filter the water had successfully reduced bacteria levels at the venue, which will also host triathlon.

They tested single and triple-layer screens -- some 20 metres (66 feet) long and three metres wide -- and found that both were effective in bringing bacteria down to safe levels although the triple screen, expected to be employed during games time, worked best.

Japanese swimmer Kida said the water was "a little stinky, and the clarity was not very good so I really want to improve the quality."

The event will be held in Odaiba, a Tokyo bay area with a backdrop of the city and the "Rainbow Bridge" that links the area to downtown.

On clear days Mount Fuji is visible and the area is also noteworthy for a replica Statue of Liberty.

© 2019 AFP