Tens of thousands of victims of last year's Gulf Coast oil spill have yet to receive compensation for their losses, a problem that a key administrator says is related to the fact that many claimants are not providing sufficient documentation to justify a payout.

Kenneth Feinberg, the lawyer who is serving as the administrator of BP's $20-billion oil spill claims fund, says a "huge number" of claimants are failing to provide basic documents such as tax returns and financial statements, which are necessary to prove the extent of their losses.

"We've notified people that they must give us some substantiation, some proof," Feinberg told CTV's Canada AM during an interview from Washington on Wednesday morning.

Feinberg recently suggested that as many as 80 per cent of the remaining claimants will not receive compensation because of documentation issues.

On Wednesday, Feinberg said it is not possible to pay a claim "based on a handshake," and the onus is on the claimants to provide the necessary paperwork.

"I really do need something and when we have it -- and I'm not asking for a great deal -- when we have some proof, we will pay the claim."

To date, the fund has paid out more than $3.5 billion in compensation to 200,000 claimants. Some 500,000 people have filed claims since the fund was created.

Feinberg said "the sheer magnitude of the spill and its impact on a whole region of the country is a challenge."

A small fraction of claimants has gone to the U.S. Coast Guard to appeal the compensation they were awarded by the fund, but Feinberg said they have not been successful in increasing their payout.

"The Coast Guard has already reviewed about 380 of my decisions and in every single case, bar none, has agreed with what I've done," Feinberg said.

Feinberg said it is also possible for unsatisfied claimants to reapply for further compensation.

The fund was just one of the costs to BP after the disastrous April 2010 explosion on the Deepwater Horizon caused an underwater well to spew millions of gallons of oil in the Gulf of Mexico. Along with the $20-billion compensation fund, BP has spent billions cleaning up the southern U.S. coast and plugging the underwater well.

With files from The Associated Press