Updated

A Bahrain judge on Thursday overturned a conviction against a prominent human rights campaigner for posting alleged anti-government comments on social media, but the activist remained jailed while appealing another prison sentence.

The decision shifts the focus among Nabeel Rajab's supporters to next month's challenge of a three-year sentence for his role in allegedly encouraging protesters to clash with security forces in the strategic Gulf kingdom, which is home to the U.S. Navy's 5th Fleet.

Bahrain has faced more than 18 months of unrest between the Sunni-led monarchy and majority Shiites who claim they face systematic discrimination. More than 50 people have died and thousands have been injured in the violence, which has included escalating attacks on police.

Two policemen were injured Wednesday in a bomb blast in a mainly Shiite area, officials said.

Rajab's attorney, Mohammed al-Jishi, said the judge threw out the conviction for the Twitter posts after a brief hearing. Rajab has already served more than half of his three-month sentence.

A statement from Bahrain's government attributed the judge's ruling to "uncertainty regarding the evidence submitted to support the lawsuit."

"It's hard to celebrate when the Bahrain authorities admit their mistake in jailing Nabeel Rajab for the tweet but keep him in prison until 2015 on other spurious charges," said Brian Dooley, director of the Human Rights Defenders Program of the U.S.-based advocacy group Human Rights First.

"They've found another way to silence him, and that's what matters," Dooley said.

Rajab's wife Sumaiya vowed to press on with an international campaign for her husband's release before the Sept. 10 appeal hearing on the three-year sentence. The court's ruling last week in that case brought rare criticism from Bahrain's allies in Washington and touched off fresh clashes around the kingdom.

In Washington, State Department spokeswoman Victoria Nuland urged Bahrain to "consider all available options to resolve this case" and take steps to open up "meaningful dialogue" with opposition groups.

Souhayr Belhassen, president of the Paris-based International Federation for Human Rights, called Rajab's acquittal a "relief" but noted that he still could face years in prison.

"At the moment, Nabeel remains in jail and Bahraini authorities continue to smother all criticism," he said.

Bahrain's rulers have offered some concessions, but not enough to satisfy main opposition groups that seek to break the monarchy's tight hold on government affairs and appointments. A government statement Thursday said King Hamad bin Isa Al Khalifa approved changes that include giving parliament greater powers over approving Cabinet posts and policies.