Breastfeeding athletes will be allowed to bring children to the Tokyo Olympics "when necessary", organisers have announced, after criticism from mothers over tough rules on bringing family to the Games.

The clarification was welcomed by some athletes as a relief, but US football star Alex Morgan slammed organisers for keeping her in the dark about the criteria.

Athletes' families are banned from attending the pandemic-postponed Games, under anti-virus rules that organisers say are needed to hold the event safely.

But Games chiefs have now made an exception for breastfeeding infants "after careful consideration of the unique situation", saying they will be allowed to accompany their mothers "when necessary".

Nursing children will not be allowed to stay at the Olympic Village though, and must stay in private accommodation such as hotels.

Organisers said in a statement late on Wednesday that it was "inspiring that so many athletes with young children are able to continue competing at the highest levels".

The statement added that they were "committed to doing everything possible to enable them to perform at the Tokyo 2020 Games".

US football star Alex Morgan, whose daughter Charlie turned one in May, said the policy did not go far enough.

"Still not sure what 'when necessary' even means," the twice-Women's World Cup winner and Olympic gold-medallist, tweeted late Wednesday.

"Is that determined by the mother or the IOC? We are Olympic mothers telling you, it is NECESSARY. I have not been contacted about being able to bring my daughter with me to Japan and we leave in 7 days."

Athletes had complained about the rules on social media, with Canadian basketball player Kim Gaucher saying she was "being forced to decide between being a breastfeeding mum or an Olympic athlete" in an Instagram video.

But she said she was "so relieved" after hearing about the U-turn.

"Woke up to some big news this morning -- Sophie can come to Tokyo. So relieved that I don't have to make this decision," she said in an Instagram video.

American marathon runner Aliphine Tuliamuk had also voiced her concern about the proposed ban.

"Sometimes I think, what if something horrible happens and I never come back home, like what if I never make it back from Tokyo?" she wrote on Instagram.

"I am sure all moms understand this exact feeling."

Organisers said "suitable measures" would be applied to infants entering Japan for the Games.