The former Gambian president Yahya Jammeh stole far more money from the state than previously thought, the new government has alleged, leaving the country with a “monstrous debt” of more than $1bn.

The autocratic former leader of the small west African country siphoned off at least $50m from social security, the country’s ports, and the national telecoms company, according to two senior ministers in new president Adama Barrow’s government.

Jammeh’s private jet was bought with $4.5m (£3.6m) taken from the state pension fund, they said. Many Gambian pensioners have been refused their pensions or given less than they are entitled to in recent years, despite having worked for the government all their lives. Some receive just $5 a month.

Jammeh was voted out of office after 22 years in power in December. He initially accepted the election result but changed his mind and refused to leave office or the country.

Under intense pressure from other African presidents and a regional military force positioned on the Gambia’s borders with Senegal, he finally fled to Equatorial Guinea in late January, taking a fleet of luxury cars with him.

It is unclear where the money allegedly taken by Jammeh is being held, if it has not been spent. Gambian ministers said they were relying on donors to help them pay salaries and would pursue Jammeh for the money, even if that meant going to his bolthole.

“This is grave. The economy is in severe distress,” said the interior minister, Mai Fatty. “During the impasse I made mention of $5m. But that is just a drop in the ocean compared to what the ministry of finance revealed.

“This is a reflection of the gross mismanagement of our economy by Yahya Jammeh and his APRC government. Their conduct amounts to total betrayal of the Gambian people, leaving behind them a monstrous debt of 48.3bn dalasis [$1bn].

“As the minister of interior, I have a responsibility to ensure that all those mismanaged public funds are recovered. We will swing into action without any delay. And ministry will leave no stone unturned, if we have to go to Equatorial Guinea, we will.”

Jammeh made “rampant” use of executive directives to obtain cash from state entities, he said. Close aides including Saul Badjie, commander of the republican guards, would allegedly take these executive directives and use them to make withdrawals.

More than $50m in income meant for Gamtel, the state telecoms company, was diverted to two accounts at the country’s central bank, which were then emptied of all but about $111,000, the minister of finance said.

Jammeh in his Hummer. He fled to Equatorial Guinea in January, taking a fleet of luxury cars with him. Photograph: Jason Florio/Corbis/Getty

About $67,000 from the ports authority was allegedly used for Jammeh’s private activities, including on gala dinners and T-shirts for his supporters, as well as payments to farmers working on his land and to his hospital, where he claimed he could cure HIV.

As well as money spent on the former president’s plane, it is claimed that another $40m was taken from the pensions fund, though it is not clear where all of it went.

Momodou Bajo, a pensioner in his 80s who worked for the Gambia’s public works department for more than 25 years, receives $30 a month, more than many, but struggles to get by.

“I have a family to feed. I cannot buy a bag of rice from my monthly pay. Three years ago, social security officials came and asked us about our situation. They promised that they were going to increase our income. But nothing has since been done,” he said.

“We will never forgive Jammeh for taking our money. We worked hard for this country, built roads and spent days and nights in the bushes.”

Amadou Sanneh, a political prisoner under Jammeh who was appointed finance minister the day after he was released from jail, said there was still much work to be done before the government could ascertain the full extent of the theft.

“There are other accounts that we have not investigated. We have to pursue every angle, every public enterprise to establish the grand total,” he said. “The end of month is coming. But we will be able to pay salaries. The donors have expressed willingness to give budgetary support. That’s for our short term needs and with that help, we will be able to sustain the government afloat.”

He said Jammeh had left the country with only $615,000 in international reserves.

“Their conduct amounts to total betrayal of the Gambian people,” he said, referring to Jammeh’s party, the APRC. “This is a truly an APRC disaster – in human rights, politically, fiscally, and monetarily, economically and financially.

“President Adama Barrow will take all steps and measures to ensure that all lost, stolen and misappropriated assets are returned to the Gambian people.”