More than $US500 million in claims have been approved under the NFL's concussion settlement, nearly a decade earlier than league officials estimated they would reach that amount.

Claims administrators in the settlement released an updated report on the concussion settlement information website saying about $US502 million ($676.63 million) had been approved in less than two years of the settlement.

The original actuarial estimates from the NFL estimated little more than $US400 million ($539.15 million) would be paid out in the first decade.

Attorneys for the retired players adjusted their estimates on the total payout of expected claims earlier this month, saying the settlement would likely reach $US1.4 billion ($1.89 billion) — almost a half billion more than the NFL originally estimated.

"We encourage all eligible former players to immediately sign up for a baseline assessment, and they can take comfort in knowing that compensation will be available for more than 60 years if they develop a qualifying condition," said Christopher Seeger, co-lead class counsel for the former NFL players.

"The fact that $US500 million in claims have been approved in less than two years proves that this settlement is fulfilling its promise to former NFL players and their families."

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Chris Nowinski, co-founder of the Concussion Legacy Foundation, was one person who responded to the figures on social media.

Nowinski, a former college footballer and WWE wrestler, questioned whether the NFL actuaries had "dramatically underestimated" the prevalence of Parkinson's disease, Lewy body dementia and chronic traumatic encephalopathy (CTE) in former NFL players.

Almost 2,000 claims have been filed in less than two years, according to the update filed on Monday.

Hundreds more of the nearly 20,500 retired players signed up to be prequalified to file claims than were expected, outpacing all previous projections.

As of Monday, the claims administrator said 7,343 medical appointments to assess neurological baselines had been made and more than 6,000 had been attended.

The settlement, which took effect January 2017, resolved thousands of lawsuits that accused the NFL of hiding what it knew about the risks of repeated concussions.

It covers retired players who develop Lou Gehrig's disease (motor neurone disease), dementia or other neurological problems believed to be caused by concussions suffered during their pro careers, with awards as high as $US5 million ($6.74 million) for the most serious cases.

AP/ABC