The chairman of the charity Oxfam International, Juan Alberto Fuentes, has been arrested in Guatemala.

He was detained as part of an investigation into a corruption scandal dating back to his time as Guatemala's finance minister.

While his arrest is not related to his role at Oxfam, it will heap further pressure on the charity, which is still reeling from revelations that its staff hired prostitutes in Haiti.

"He has assured us that he has co-operated fully with the investigation in the confidence he did not knowingly transgress rules or procedures," Ms Byanyima said.

It comes as Oxfam faces scrutiny over a scandal in Haiti in 2011, where senior aid workers - including the country director Roland Van Hauwermeiren - allegedly paid for sex. Some of the claims say the women may have been underage.

The scandal has forced the resignation of the organisation's deputy chief executive, Penny Lawrence, and shaken public confidence in both Oxfam and other charities.

In Guatemala, Mr Fuentes is among 10 top former government officials arrested on Tuesday, including former President Álvaro Colom.

The ex-officials are being investigated in connection with a public bus system, which was developed during Mr Colom's government between 2008 and 2012.

No details have been given of the charges those arrested may face.

The investigation is being led by the International Commission against Impunity in Guatemala (CICIG). The commission is a United Nations body created in 2006 designed to strengthen the country's rule of law.

Apart from Mr Colom, those arrested are the former ministers of finance, interior, education, defence, employment, economy, health, culture and the environment, reports BBC.