Saudi-led forces sent to Bahrain to help crush anti-government protests will remain even after emergency rule is lifted next month, the head of the kingdom's military said in a move that is likely to deepen regional tensions with Iran.

And Bahrain's crackdown on opposition continued Thursday when a special security court sentenced a protester to 15 years in prison. Twenty-one others had their cases continued by the court, which has ordered executions in some previous cases.

Shiite power Iran has condemned the 1,500-strong Gulf Arab force in Bahrain as an "occupation" by Sunni states against Bahrain's Shiite majority, which has faced waves of arrests and deadly crackdowns after beginning protests for greater rights three months ago.

In response, Gulf leaders have sharply warned Iran to stay out of their affairs and accused Bahrain's protesters of having links to groups such as the Iranian-backed Hezbollah militants in Lebanon.

The Bahrain military commander, Sheik Khalifa bin Ahmed Al Khalifa, also threatened even harsher crackdowns if demonstrators return to the streets in the strategic U.S. ally, which is home to the Navy's 5th Fleet.

"I say to those who did not get the message, 'If you return we will come back, stronger this time,'" Sheik Ahmed was quoted as saying late Wednesday by the official Bahrain News Agency.

He further claimed that protesters were "given pills which affected their minds and made them do unusual things" — a new allegation that echoed assertions by Libyan leader Moammar Gadhafi that his opponents included young people on hallucinogenic pills placed "in their coffee with milk, like Nescafe."

Meanwhile, in a special security court set up under martial law-style rule imposed in March, the expected resumption of a trial against 21 opposition leaders and human rights activists was adjourned until May 16. The activists are accused of plotting against the state and having links to foreign factions — an apparent reference to Hezbollah. Fourteen of the suspects are in custody and the rest are being tried in absentia.

Among the 21 charged are a Swedish citizen and a person with a Swedish residency permit, Swedish foreign ministry spokesman Teo Zetterman said. The Swedish citizen also has Bahraini citizenship, Zetterman said, though the dual citizenship is not recognized by Bahrain, making it hard for the Swedish embassy in Abu Dhabi to assist.

Separately, the state news agency said Thursday that authorities had released 24 detained doctors and nurses pending trial on charges that include seeking to topple the state. Earlier this week, Bahrain's justice minister said that a total of 23 doctors and 24 nurses would face trial.

Also Thursday, the security court convicted another opposition supporter on charges of attempted murder of a police officer and participation in a protest aimed at disrupting public order, a report by the state-run Bahrain News Agency said Thursday. It added that the protester, Hamad Yousef Kazim, was sentenced to 15 years in prison. Kazim's court-appointed lawyer can appeal the sentence, the report said.

Bahrain's king said the emergency rule will be lifted June 1. But the military chief's statements suggest a heavy security presence will remain along with the Saudi-led troops.

Bahrain's Shiites comprise about 70 percent of the population, but claim widespread discrimination and abuses at the hands of the 200-year-old ruling Sunni dynasty.

At least 30 people have been killed since the protests began in February, inspired by revolts against autocratic leaders in Tunisia and Egypt. Hundreds of protesters, opposition leaders, human rights activists, athletes and Shiite professionals such as doctors and lawyers have been detained.