Bangkok: By the time Ferdinand Marcos fled a popular uprising on the streets of Manila, he had amassed a fortune estimated at $US13.5billion ($17billion).

His annual salary during 21 years as president of the Philippines never rose above $US13,500.

Now, 31 years, later Marcos' heirs have offered to return some of the wealth they claimed never existed.

"They [the Marcos family] told me they'll open everything and probably return what is uncovered," incumbent President Rodrigo Duterte told reporters, after being approached by an unnamed Marcos family member.

"They are ready to bring it back…including a few gold bars," he said.

But Duterte, a political ally of the late president's son, Ferdinand "Bongbong" Marcos jnr, said the amount offered was not "Fort Knox", referring to the vault that stores United States gold reserves.  

Duterte attempted to justify one of the largest plunderings of state coffers in modern times, claiming that Marcos, who died in Honolulu in 1989, had kept the wealth because he was just protecting it for the economy.

Marcos family members have enjoyed a remarkable political comeback in recent years and are grooming Bongbong to become the next president.

After running for the vice-presidency last year, Bongbong is challenging the vote in court in a fight that could install him in the country's second-highest office.

When the late Ferdinand Marcos and his family fled the country in 1986, the 1220 pairs of shoes his wife Imelda left behind became a symbol of kleptocracy​ in a nation where many still walked around barefoot in abject poverty.

Over the decades, the shoes, stored in a museum, were damaged by termites, storms and neglect, as staff appointed to a Presidential Commission on Good Government initiated legal action across the world to track down the Marcos fortune.

They recovered $US4.4billion from Swiss bank accounts, shares, real estate, paintings and jewellery, including three of the world's top jewel collections that included "diamond studded tiaras, necklaces, brooches, earrings, belts and other gems".

But powerful Philippine politicians are now demanding the return of the remaining billions.

Senate President Aquilino Pimentel, a staunch Duterte ally, said it isn't enough that the Marcos family only return part of their ill-gotten wealth.

"Everything that was stolen should be returned…it's not like we will be satisfied just with crumbs," he said.

Senator Francis Pangilinan said the intentions of the Marcos family should not be taken at face value.

"Aside from returning the ill-gotten wealth the Marcoses should also apologise for atrocities committed during martial law," he said.

But BongBong denies his parents ever committed any crime. He seems assured of Duterte's backing in his bid for the presidency at the next elections.

Despite ordering a war on drugs that has left more than 12,000 mostly poor Filipinos dead, Duterte's popularity remains high among the country's 100 million citizens. 

Duterte stunned his nation in November when he allowed the late president's body to be buried in the national Heroes' Cemetery. Critics said Marcos should have been denied such an honour.

The Marcos family has again become one of the Philippines' most powerful political clans.

Imelda, now 88, has been a congresswoman since 1995. Attempting to justify her extravagant lifestyle, she has often claimed her late husband's fortune came from a recovered treasure hidden by a Japanese general during World War II. 

The oldest Marcos daughter, Imee, is Governor of northern Illocs Norte, the family's stronghold.  

After winning office in a landslide last year Duterte thanked the Marcos family for their financial support during his campaign.