WASHINGTON -- As vast offshore oil fields generate hundreds of millions of dollars for tiny Equatorial Guinea, there are few signs of the petroleum boom in the impoverished West African nation.

Most of the population lives on about a dollar a day, and a U.S. State Department report found "little evidence that the country's oil wealth is being devoted to the public good."

So where has the money gone?

That has been declared a "state secret" by Equatorial Guinea's ruler, Brig. Gen. Teodoro Obiang Nguema Mbasogo.

But the Guinean ambassador to the U.S. and other sources close to Obiang say the country's oil funds are held in an account at Riggs Bank in Washington.

According to several of those sources and others familiar with the account, more than $300 million of the country's energy earnings has been deposited in the account by international oil companies active in Equatorial Guinea, including ExxonMobil Corp. and Amerada Hess Corp. The money is under the direct control of Obiang, the sources say.

The arrangement has raised concerns at the International Monetary Fund, where officials have refused to provide assistance to Equatorial Guinea until Obiang accounts for his country's oil money and have urged him to transfer it to its home treasury. It has also complicated efforts by the Bush administration to improve ties with the country, which soon will become sub-Saharan Africa's third-largest oil producer after Nigeria and Angola. Critics say the administration should not embrace Obiang's regime until it improves its human rights record and implements anticorruption reforms.

Oil company payments into offshore government accounts are not illegal, and several other African energy-producing countries have similar arrangements. But they are sharply criticized by international financial institutions and anticorruption groups, because they increase the possibility for diversion of oil revenue into private bank accounts of well-placed officials.

Alejandro Evuna Owono, a top aide to Obiang, denied that the government was secretive about oil revenue. "The IMF and the World Bank know national production figures, but we can use the money as we see fit," he said. "We are an independent country and they cannot interfere with the management of those resources."

Owono would not say whether the government held its oil monies at Riggs. But Guinean Ambassador Teodoro Biyogo Nsue, who is Obiang's brother-in-law, mentioned that oil revenue was held at Riggs during a presentation on Equatorial Guinea late last year at the Center for Strategic and International Studies in Washington, according to three people who attended.

Multiple sources, including another Guinean government official, have since told the Los Angeles Times about the Riggs account. Several sources familiar with the account said it was controlled exclusively by Obiang and its balance has ranged from $300 million to $500 million during the last two years.

An ExxonMobil spokeswoman declined to comment on payments it makes to Equatorial Guinea, citing a confidentiality agreement with the country. Amerada Hess and Riggs did not return phone calls.

The bank has provided mortgages on one of Obiang's two luxury homes in Maryland and on an official residence for Nsue in Virginia. A Riggs banker assisted Obiang's brother -- accused in State Department reports of ordering the torture of political prisoners -- in the purchase of a home in Virginia. The banker vouched for Obiang's brother as a "valued customer" in correspondence with the seller's agent.

Concerns about Equatorial Guinea are rooted in a history of petroleum-fueled corruption in its neighboring nations.

Angola ranks 161st out of 173 countries on the United Nations' Human Development Index, which ranks nations according to their citizens' quality of life, and its president, Jose Eduardo dos Santos, is believed to have accumulated a vast personal fortune. The London-based group Global Witness, which investigates the oil business in Africa, estimates that at least $1.4 billion of Angola's energy revenue disappeared last year.

The situation in Nigeria is equally grim. In April, the family of former strongman Gen. Sani Abacha agreed to return $1 billion that he stole between 1993 and 1998. That was only about one-quarter of the money that the current Nigerian government accuses Abacha of embezzling during his reign.

African countries provide the United States with about 15% of its oil, nearly as much as Saudi Arabia. That figure could grow to 25% by 2015, according to a study by a U.S. intelligence panel. Urged on by the oil industry, the administration hopes to use African oil to reduce the United States' dependence on Middle Eastern oil.

"Without pressing forcefully for improvements in governance and human rights, the U.S. could end up coddling a number of oil-rich dictatorships," says Arvind Ganesan, director of the business and human rights program at Human Rights Watch.

Oil company officials have said they are obliged to accept the terms set by the governments in the countries where they operate in order to obtain exploration permits.