DUBAI, United Arab Emirates (AP) — Activists on Wednesday asked the United States to suspend a diplomatic visa for a Bahrain prince over allegations that he tortured prisoners during the island kingdom's 2011 Arab Spring protests.

Bahrain long has denied the allegations against Prince Nasser bin Hamad Al Khalifa. However, as his prominence has risen in recent years, so too have the lingering memories of the 2011 demonstrations, with Bahrain cracking down on all dissent for more than a year now.

Meanwhile, Bahrain announced it had charged the imprisoned leader of what was the country's largest Shiite opposition group over allegedly communicating with Qatar, a country Manama now boycotts with three other Arab nations.

The group Americans for Democracy and Human Rights in Bahrain sent letters to the U.S. State Department and Defense Department over their concerns about Prince Nasser. They asked that his visa be suspended, as well as any defense cooperation with him. The prince was appointed in September to Bahrain's Supreme Defense Council, its highest military authority.

"We are gravely concerned by the U.S. government's open association with Bahraini military officials like Sheikh Nasser, whose leading role in defense procurement and record of malfeasance render him a particularly high risk for further corruption and abuse," wrote Husain Abdulla, the executive director of Americans for Democracy and Human Rights in Bahrain.

Bahrain's government and U.S. officials did not respond to a request for comment.

The island kingdom remains a crucial part of American military strategy in the Persian Gulf by hosting the U.S. Navy's 5th Fleet. The Trump administration also has approved a multibillion-dollar sale of F-16 fighter jets to Bahrain without the human rights conditions imposed by the State Department under President Barack Obama.

Bahrain's Shiite majority and others demonstrated in 2011 to ask the island's Sunni rulers for more political freedoms. Bahrain put down the protests with the help of Saudi and Emirati forces.

For more than a year, Bahrain has been targeting journalists, activists, Shiite religious leaders and political parties. Some activists have escaped into exile while others have been imprisoned. Independent news gathering there has grown more difficult, with the government refusing to accredit two Associated Press reporters and others.

Amid the crackdown, local Shiite militant groups have carried out several attacks on security forces.

Wednesday's letters come after Prince Nasser attended an event in September at the Los Angeles-based Simon Wiesenthal Center. Rabbis at the event said Bahraini King Hamad bin Isa Al Khalifa told them the longtime boycott of Israel by Arab countries should end.

As the letters were publicized, Bahrain prosecutors issued a statement Wednesday saying they had questioned and charged Sheikh Ali Salman, the secretary-general of the now-dissolved Al-Wefaq political group. Salman, a central figure in the 2011 protests, is already serving a nine-year sentence on charges including incitement and insulting the Interior Ministry.

A lawyer for Salman did not respond to a request for comment, though activists said the cleric denied all charges against him.

At the time of the protests, Qatar had done some negotiations behind the scenes to ease tensions on the island. Now amid the monthslong boycott of Qatar, intercepted phone calls from that time have been played on Arab satellite channels and have pointed to as a sign of collusion between Doha and the protesters.

There has been bad blood for decades between Qatar and Bahrain, which have included disputes over islands in the Persian Gulf. On Tuesday, Bahrain's Interior Ministry announced Qatari nationals and residents there must obtain a visa prior to arrival in Manama, breaking the visa-free travel arrangements shared by Gulf Arab nations.

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