A bomb explosion in Bahrain killed three police, including an Emirati, during confrontations with "rioters" near Manama, in the bloodiest attack on the security forces since they crushed the 2011 uprising.
Witnesses reported hearing a blast in the Shiite-populated village of Daih, on the outskirts of the Bahraini capital, as the police fired tear gas and buckshot to break up a demonstration.
UN Secretary General Ban Ki-moon condemned the deadly bomb attack
and sent his condolences to the victims' family, his spokesman said.
"Such acts of violence cannot be justified by any cause... He urges all Bahrainis to come together to create a conducive environment for promoting reconciliation," the UN leader's spokesman added.
The explosion is the most serious attack on the security forces in terms of casualties since the Shiite majority led an Arab Spring-inspired uprising in February 2011 against the Al-Khalifas.
Clashes frequently erupt near Manama between security forces and Shiite protesters demanding the Sunni ruling Al-Khalifa dynasty surrender its grip on all key cabinet posts in favour of an elected government.
On Monday, "three police personnel died in a terror blast in Daih while police were dispersing rioters," Bahrain's interior ministry said on Twitter.
And the interior ministry in the United Arab Emirates said an officer from its police force was among the dead.
The Shiite-dominated opposition swiftly condemned Monday's bombing, stressing any political demands had to be voiced in a "peaceful" manner.
Emirati First Lieutenant Tariq al-Shehi died along with two members of the Bahraini police force "while performing his national duty of maintaining order," said the UAE interior ministry.
He is the first Gulf officer reported to have been killed since forces from the region rolled into Bahrain in March 2011 to boost the kingdom's security forces, which later quelled the month-long uprising.
Bahrain has always maintained the Gulf force did not take part in confrontations with protesters and have been deployed to protect vital installations.
The UAE ministry said Shehi was part of a force established as part of common Gulf security pact.
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- Opposition 'regrets' bloodshed -
Its Bahraini counterpart had said earlier that security forces dispersed a "breakaway group of thugs who diverted from a funeral route in Daih to riot".
On Sunday, a policeman was wounded in a blast in the Shiite village of Akr al-Sharqi, said the interior ministry.
A policeman died of wounds sustained in a bomb blast in the village of Dair, north of Manama, during the commemoration of third anniversary of protests on February 14, the interior ministry said at the time.
Six opposition groups, led by the Al-Wefaq main Shiite formation, said they "regretted having casualties regardless of which side they belonged to, including security forces".
"The sanctity of the blood applies to every human being," said a joint opposition statement.
It called on supporters to "adhere to peaceful means and condemn and disclaim criminal acts claimed by the so-called Al-Ashtar Brigades or Resistance (brigades) or any other party that claims responsibility for bomb attacks and violence."
Al-Ashtar Brigades has reportedly claimed responsibility for bomb attacks in the kingdom, including a July bombing outside a Sunni mosque.
In March 2011, Bahrain was backed by security forces mainly from Saudi Arabia when it brutally crushed the Shiite-led pro-democracy protests inspired by the Arab Spring uprisings.
Three years on, the kingdom remains deeply divided, with persistent protests that ignite clashes with police, scores of Shiites jailed on "terror" charges, reconciliation talks deadlocked and sectarian distrust simmering.
The International Federation for Human Rights says at least 89 people have been killed.
Last year the authorities increased the penalties for those convicted of violence, introducing the death penalty or life sentences in cases which resulted in deaths or injuries.