Bahrain has detained four people for defaming the island's king with Twitter posts in a case that is likely to test the limits of expression in the violence-wracked Gulf nation.

Similar prosecutions across the region have brought strong criticism from media freedom groups.

Bahrain has come under intense scrutiny for its crackdowns on opposition protesters since February 2011. The island's majority Shiites have battled a greater political voice in the Sunni-ruled kingdom, which is home to the U.S. Navy's 5th Fleet.

Bahraini authorities did not give further details on the suspects or the contents of their Web posts.

The official Bahrain News Agency said late Wednesday the four will be held for seven days pending trial.

Twitter and other social media have become key tools of Bahrain's opposition groups.