The office of Switzerland's attorney general is helping authorities in Oman investigate alleged corruption in the sultanate's national oil company and oil ministry, a Swiss law enforcement source familiar with the matter said.

Swiss authorities are involved because transactions related to the probe involved Swiss financial institutions, the source said without identifying any of the institutions.

Oman's Sultan Qaboos bin Said Al Said has waged an anti-graft campaign in the wake of mass protests held in several cities in 2011 in the wake of the "Arab Spring" uprisings across the Arab world that year.

Last year, the Court of First Instance in Muscat, Oman's capital, sentenced the CEO of state-owned Oman Oil Company to 23 years in jail for accepting bribes, abuse of office and money laundering.

Speaking at an international prosecutors' convention on Monday in Zurich, Swiss Attorney General Michael Lauber said publicly that his office was cooperating on big cases with "colleagues" in Brazil and Oman but did give further details.

Swiss authorities have been collaborating with Brazil on its investigation into Brazilian oil firm Petrobras as well as Latin America's largest engineering and construction company, Odebrecht SA, and its subsidiaries.  - Reuters