MUSCAT (Reuters) - A court in Oman on Monday ordered the permanent closure of a newspaper which had reported on alleged corruption within the judiciary, and jailed three of its journalists on charges that included undermining the prestige of the state.

Witnesses at the court said Ibrahim al-Mamari, editor-in-chief of the privately owned Azamn newspaper, and his deputy and managing editor Youssef al-Balushi, were jailed for three years and fined 3,000 rials ($7,800) each. A third journalist was jailed for a year.

Azamn had extensively covered a series of corruption cases in 2014 in which several company executives were convicted.

In Washington, State Department spokesman Mark Toner said the United States was very concerned about the court decisions and the U.S. ambassador had conveyed the concern to Oman's government at a senior level.

"We support freedom of expression and maintain that societies are strengthened when their citizens are able to voice their opinion," Toner told a daily news briefing. He said U.S. embassy staff had attended the court hearing.

Omani authorities suspended the newspaper in August for a month after detaining the three journalists. In a statement which did not mention Azamn by name, they said it had exceeded the limits of free speech and "drifted into ... harming one of the pillars of the state, the judiciary."

Amnesty International said at the time it appeared the journalists were being punished for carrying out "legitimate journalistic work".

The Muscat Court of First Instance found Mamari and Balushi guilty on four charges, including undermining the prestige of the state, disturbing public order, misusing the internet, and publishing details of a personal status case.

It also convicted Balushi of slander and of publishing a report in violation of a ban imposed by the Information Ministry on any reporting related to Mamari's arrest.

The third journalist, Zaher al-Abri, was sentenced to one year in jail and fined 1,000 rials.

The court set a bail of 50,000 rials for Mamari and Balushi in case they decided to appeal the rulings, while Abri's bail was set at 5,000 rials.

Five years ago a court ordered the newspaper closed down for one month and Mamari and a reporter were given five-month suspended jail sentences for insulting the justice minister and other officials.

(Reporting by Fatma Al Arimi; Additional reporting by Arshad Mohammed in Washington; Writing by Sami Aboudi; Editing by Dominic Evans and Sandra Maler)