Three British businessmen have admitted their roles in paying multimillion-dollar bribes to officials in nine countries over 17 years, American prosecutors have said.

Two brothers from the Ahsani family, who ran the energy consultancy Unaoil, have pleaded guilty to facilitating the payment of bribes between 1999 and 2016 to officials in Africa and the Middle East.

They laundered money in order to conceal the bribes and caused the destruction of evidence to obstruct investigators in the US and elsewhere, according to US prosecutors.

Cyrus Ahsani, 51, and his brother Saman, 46 – described by US investigators as being British residents – pleaded guilty to conspiring to facilitate bribes on behalf of multinational firms to secure oil and gas contracts. The engineering company Rolls-Royce is among the firms that have said they hired Unaoil.

Steven Hunter, a 50-year-old British resident and former Unaoil business development director, also pleaded guilty to bribery.

The announcement of the guilty pleas – made on Wednesday evening by the US Department of Justice (DoJ) – is the latest development in a wide-ranging international criminal investigation which is expected to continue.

Anti-corruption prosecutors in a series of countries – including the UK’s Serious Fraud Office – began investigating bribery allegations involving Unaoil three years ago.

The SFO was given extra funds by the British government to investigate the allegations, which were brought to light after the leak of thousands of the firm’s emails to journalists at Australia’s Fairfax media.

In July, a former Unaoil associate, Basil Al Jarah, 70, from Hull, pleaded guilty in a London court to five counts of conspiring to give corrupt payments to public officials to secure commercial contracts in Iraq. It was the first guilty plea to result from the SFO’s investigation. He has yet to be sentenced. Three other men are due to go on trial in London in January.

In its announcement on Wednesday, the US prosecutors said the Ahsani brothers had managed Monaco-based Unaoil.

“From approximately 1999 to 2016, the Ahsanis conspired with others, including multiple companies and individuals, to make millions of dollars in bribe payments to government officials in Algeria, Angola, Azerbaijan, the Democratic Republic of Congo, Iran, Iraq, Kazakhstan, Libya and Syria,” they said.

The two brothers and Hunter pleaded guilty to one count of bribery and will be sentenced next year. Hunter admitted facilitating bribery payments to Libyan officials between about 2009 and 2015.

Tom Martin, a former SFO prosecutor, has started legal proceedings against the agency after it sacked him for allegedly swearing at an FBI agent in a pub. He had been leading the SFO’s investigation into Unaoil.

In legal papers, he has alleged that Saman Ahsani and the US DoJ agreed to make a joint complaint against him in order to remove him as the case controller of the investigation and to prevent the SFO from extraditing Ahsani. His employment tribunal is due to be heard in December.