The sweetener aspartame is a "possible carcinogen" but it remains safe to consume at already agreed levels, two groups linked to the World Health Organisation (WHO) have declared.

The rulings are the outcome of two separate WHO expert panels, one of which flags whether there is any evidence that a substance is a potential hazard, and the other which assesses how much of a real-life risk that substance actually poses.

Aspartame is one of the world's most popular sweeteners, used in products from Coca-Cola diet sodas to Mars' Extra chewing gum.

In a press conference ahead of Friday's announcement, the WHO's head of nutrition, Francesco Branca, tried to help consumers make sense of the seemingly conflicting declarations, especially those who seek out artificial sweeteners to avoid sugar.

"If consumers are faced with the decision of whether to take cola with sweeteners or one with sugar, I think there should be a third option considered - which is to drink water instead," Branca said.

In its first declaration on the additive, announced early on Friday, the Lyon-based International Agency for Research on Cancer (IARC) said aspartame was a "possible carcinogen".

That classification means there is limited evidence that a substance can cause cancer.

It does not take into account how much a person would need to consume to be at risk, which is considered by a separate panel, the WHO and Food and Agriculture Organisation (FAO) Joint Committee on Food Additives (JECFA), based in Geneva.

After undertaking its own comprehensive review, JECFA said on Friday it did not have convincing evidence of harm caused by aspartame, and continued to recommend that people keep their consumption levels of aspartame below 40mg/kg a day.

It first set this level in 1981, and regulators worldwide have similar guidance for their populations.

Several scientists not associated with the reviews said the evidence linking aspartame to cancer is weak. Food and beverage industry associations said the decisions showed aspartame was safe and a good option for people wanting to reduce sugar in their diets.

The WHO said the existing consumption levels meant, for example, a person weighing between 60-70kg would have to drink more than 9-14 cans of soda daily to breach the limit, based on the average aspartame content in the beverages - around 10 times what most people consume.

"Our results do not indicate that occasional consumption could pose a risk to most consumers," said Branca.

He said the WHO is not urging companies to remove aspartame from their products entirely, but is instead calling for moderation from both manufacturers and consumers. Earlier this year, the WHO said there was no evidence that sweeteners helped with weight control, which the industry has disputed.

"We have raised a flag," said Branca.

It was earlier reported in June that the IARC would put aspartame in group 2B as a "possible carcinogen" alongside aloe vera extract and traditional Asian pickled vegetables.

The IARC panel said on Friday it had made its ruling based on three studies in humans in the United States and Europe that indicated a link between hepatocellular carcinoma, a form of liver cancer, and sweetener consumption, the first of which was published in 2016.

It said limited evidence from earlier animal studies was also a factor, although the studies in question are controversial. There was also some limited evidence that aspartame has some chemical properties that are linked to cancer, the IARC said.

"In our view, this is really more a call to the research community to try to better clarify and understand the carcinogenic hazard that may or may not be posed by aspartame consumption," said Dr Mary Schubauer-Berigan, acting head of the IARC Monographs program.

Scientists with no links to the WHO reviews said the evidence that aspartame caused cancer was weak.

The WHO conclusion "once again affirms that aspartame is safe", said Kate Loatman, executive director of the International Council of Beverage Associations.