Bahrainis wielding knives and sticks attacked Shiite villagers overnight, witnesses said Wednesday, as pressure grew for this month's Formula One race in the Sunni-ruled Gulf state to be cancelled over the unrest.
Washington said it was "deeply concerned" about the upsurge of violence in the tiny but strategic archipelago which is the home base of the US Fifth Fleet.
A day after a petrol bomb injured seven policemen, men from Sunni neighbourhoods who answered an Internet call to avenge the attack converged on Shiite-populated villages near Manama late Tuesday, according to witnesses.
"I saw hundreds of men carrying knives, sticks and other sharp objects," on the outskirts of several Shiite villages, said one witness who identified himself only as Hussein.
"They were stopping cars and asking passengers where they lived in order to determine what sect they belonged to," he told AFP by telephone, referring to the Sunni and Shiite communities in Bahrain.
Al-Wefaq, the Gulf kingdom's largest Shiite opposition group, said the attackers were in civilian clothes and "beat up" the residents of Shiite villages.
A statement by the group accused security forces of failing to stop the attacks.
"The security forces did not carry out their duty. They did not disperse the (assailants) or prevent them from attacking citizens," the statement said, adding that the authorities must "deal with these militias."
But Bahrain's interior ministry said police "prevented" a group of unidentified assailants from entering the Shiite village of Al-Nuweidrat on the outskirts of the capital.
In a statement, the ministry said assailants "attacked 24-hour shops and destroyed two cars," after holding an "illegal gathering," without mentioning the reported attacks by Sunnis on Shiite neighbourhoods.
Four people have been arrested in connection with the police bombing, described by the interior minister as a "dangerous development."
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In a statement on state news agency BNA, Lieutenant-General Sheikh Rashid bin Abdullah al-Khalifa said the attack was "part of a campaign of violence and sabotage," adding the government would show "zero tolerance against terrorists."
The White House condemned attacks on police but also called on security forces to show restraint.
"The United States continues to be deeply concerned about the situation in Bahrain, and we urge all parties to reject violence in all its forms," White House spokesman Jay Carney said in a statement.
Tensions are high in Bahrain over the health of prominent Shiite activist Abdulhadi al-Khawaja, who is on hunger strike and who has been sentenced to life imprisonment after being convicted of plotting to overthrow the Sunni monarchy during Shiite-led anti-government protests last year.
Shiite youth groups have been organising protests almost every evening in solidarity with Khawaja.
On Tuesday, Denmark's Prime Minister Helle Thorning-Schmidt said Khawaja, who is a dual Danish-Bahraini citizen, is in "very critical" condition.
Anger is also mounting over a decision by motorsport chiefs to go ahead with the Grand Prix as scheduled on April 22, although F1 teams were still expecting the race to be cancelled, according to some reports.
The youth group of the "Revolution of February 14" has called for "three days of rage" in Bahrain from April 20 to 22, and also launched a campaign on Twitter to cancel the race.
On Tuesday, F1 supremo Bernie Ecclestone admitted this week that he was unable to force teams, reluctant to race in Bahrain over security concerns, to participate in the event.
A government statement released Wednesday assured visitors the race would be held in "safe and stable conditions," adding the bouts of unrest are "infrequent" and happen "in distant villages."
Last year's Grand Prix event was initially postponed and later cancelled following the February and March protests inspired by Arab uprisings.
John Yates, a former assistant commissioner in the London Metropolitan Police Service who is an adviser to Bahrain's interior ministry on policing, has said there "is nothing that in any way warrants for the race to be postponed."