Story highlights NEW: U.S. State Department "deeply disturbed" by sentences

U.S. State Department "deeply disturbed" by sentences The doctors are accused of trying to overthrow the Bahraini government

Most of the doctors were sentenced to 15 years in prison

The charges "are simply ludicrous," an Amnesty International official says

A group of 20 doctors who were detained during this year's protests in Bahrain have been convicted of attempting to overthrow the government and hit with lengthy prison sentences, authorities and a human rights group said Thursday.

Thirteen of the physicians were sentenced to 15 years in prison, two for 10 years and five for five years, said military prosecutor, Col. Yussef Rashid Flaifel.

The U.S. State Department, "deeply disturbed" by the sentences, said the Bahraini government should provide fair trials, access to attorneys and judicial transparency.

Deputy spokesperson Mark Toner said the United States was "concerned about trials of civilians, including medical personnel, in military courts and the fairness of those proceedings."

Charges against the doctors included possession of unlicensed weapons, inciting the overthrow of the government, provoking sectarian hatred and forceful occupation of a public building, officials said. Prosecutors have alleged that, at the height of the protests earlier this year, the accused medical personnel refused to help patients at Salmaniya Medical Complex, the main hospital in the capital city, Manama.

"The Salmaniya Hospital was used as a coordination center for three weeks for protests calling for the overthrow of the government. Those doctors who have been found guilty were charged with abusing the hospital for political purposes," a government spokesman said. "Nobody is above the law."

This month, the doctors were released on bail after reports that several of them had gone on hunger strikes.

At the time, human rights activists hoped that their release was good sign, though the charges against the doctors still stood.

But Thursday, Amnesty International issued a statement decrying the court's decision.

"These are simply ludicrous charges against civilian professionals who were working to save lives amid very trying circumstances," said Philip Luther, deputy director of Amnesty International's Middle East and North Africa program. "It appears that the real reason for targeting these health workers was the fact that they denounced the government crackdown on protesters in interviews to international media."

"We've repeatedly said that Bahraini authorities should never have used military courts to prosecute ordinary civilians, including doctors, teachers and human rights activists," he added.

An attorney for the doctors said the group will appeal the verdict next month in a Bahraini civilian appellate court.

Protests have lingered in Bahrain for months despite a crackdown by the kingdom's Sunni monarchy, backed by troops from Saudi Arabia and the United Arab Emirates. More than 30 people have been killed during the demonstrations, in which activists say Bahraini security forces used live ammunition against protesters.

Opposition groups say more than 1,000 people -- mainly Shiites -- have been detained and more than 2,000 have lost their jobs for allegedly taking part in the protests.