MANAMA – The United Nations warned on Thursday of "shocking and illegal" abuses in Bahrain where the US-backed Sunni Muslim rulers are waging a bloody crackdown on Shiite-led protesters.

The violence in the strategic Gulf kingdom has alarmed Washington and sparked furious condemnation from Iran, Shiite leaders in Iraq and the Hezbollah militia in Lebanon.

Dissidents were rounded up at gunpoint in midnight raids and armed police stood outside Manama's main hospital, amid reports the authorities were beating doctors and denying treatment to the wounded.

Fresh clashes erupted in at least one Shiite village outside the capital, a day after five people were killed when security forces crushed a month-old pro-democracy sit-in at Manama's Pearl Square.

UN rights chief Navi Pillay said any takeover by the security forces of hospitals and medical facilities was a "blatant violation of international law."

"There are reports of arbitrary arrests, killings, beatings of protesters and of medical personnel, and of the takeover of hospitals and medical centres by various security forces," she said.

"This is shocking and illegal conduct."

Five hardline Shiite activists and one Sunni dissident were arrested overnight after the army imposed a curfew on parts of Manama using its powers under newly imposed martial law, opposition sources said.

Bahrain's military confirmed it had arrested a number of people for crimes including sedition, murder and having contact with foreign states, but gave no details.

"Four men arrived around two in the morning. One of them put a revolver to my husband's temple and took him away without even giving us time to call his lawyer," Farida Gulam, the wife of detained leftist Ibrahim Sharif, told AFP.

Opposition MP Khalil al-Marzouk also said that Doctor Ali al-Ekri, who had been accused on state TV of spreading "fabrications" about conditions at Salmaniya hospital, was arrested there on Thursday.

Security forces firing tear gas and shotguns cleared out a pro-democracy tent city at Pearl Square on Wednesday in the worst day of violence since activists took to the streets last month.

The opposition said three demonstrators were killed in the raid, while the government said two police died in hit-and-run attacks by opposition motorists.

Hundreds have been wounded in clashes in recent days, witnesses and medics said.

US President Barack Obama, whose country is a close ally of Bahrain, called King Hamad to express "deep concern," while British Prime Minister David Cameron urged the monarch to pursue "reform, not repression."

The main opposition is demanding a constitutional monarchy, the resignation of the government and an end to repression in Bahrain.

King Hamad declared a three-month state of emergency on Tuesday a day after more than 1,000 armoured troops from Saudi Arabia and the United Arab Emirates entered Bahrain to help restore security.

The sectarian tensions have given rise to fears among Bahrain's Sunni-led Arab neighbours that Iran, the Shiite power a short boat ride away across the Gulf, is seeking to foment unrest in the kingdom.

US Secretary of State Hillary Clinton called the situation in Bahrain "alarming" and criticised Washington's Gulf state allies for heading down "the wrong track" of military intervention.

Bahrain is the home of the US Fifth Fleet and is a regional financial hub.

And Russia on Thursday called the Gulf military intervention in Bahrain a "road to nowhere."

"The use of force to resolve the situation, even if it involves mechanisms of regional cooperation, can hardly help calm the situation," news agencies quoted foreign ministry spokesman Alexander Lukashevich as saying.

"Military intervention, either on a regional or broader level, is a road to nowhere."

The US Pentagon has authorised the departure of non-emergency Defence Department staff, while Britain said it was chartering planes to help evacuate its citizens.

Police and troops have fanned out across Manama, and Shiite villages in the surrounding countryside were reportedly sealed off. But there were few signs of the Gulf forces, which were keeping a low profile.

Security forces opened fire with tear gas and shotguns to scatter several hundred protesters who rallied in the Shiite village of Deih, west of Manama, Bahrain Centre for Human Rights chief Nabeel Rajab said.

It was not known if there were any casualties.

In Manama, tanks and troops guarded key areas and banks and big businesses were closed. The army eased a dusk-to-dawn curfew imposed on Wednesday.

Health Minister Nizar Baharna, a Shiite, quit after police raided Manama's Salmaniya hospital in what state television reported as an operation to "cleanse" it of "saboteurs."

Twelve Shiite judges also stepped down in protest at the "excessive use of force."

World oil prices rallied on Thursday, with New York's main contract, light sweet crude for delivery in April leaping $2.92 to $100.90 per barrel.

In London late afternoon trade, Brent North Sea crude for April surged $3.41 to $114.01 a barrel.