Bahrain has executed three men found guilty of killing three policemen, including an Emirati officer, prosecutors in the island kingdom said.

Abbas al-Samea, 27, Sami Mushaima, 42, and Ali al-Singace, 21, faced the firing squad on Sunday, a week after a court upheld their death sentences over a bomb attack in March 2014, the prosecution said in a statement carried by the official BNA state news agency.

They are the first in six years in the Gulf kingdom, according to London-based human rights group, Reprieve, which had warned on Saturday against the move.

Activists reacted with rage, calling it a "black day" and posting images of protesters clashing with police on social media. 

"This is a black day in Bahrain's history. It is the most heinous crime committed by the government of Bahrain and a shame upon its rulers," said Sayed Ahmed Alwadaei of the Bahrain Institute for Rights and Democracy.

Amnesty International said the executions were carried out "after an unfair trial and despite claims from the men that they were tortured in custody".

Bahrain denies practising torture.

The punishment is likely to further poison ties between Bahrain, its ally Saudi Arabia and Iran, which Bahrain accuses of fomenting unrest, including by supplying arms to Shia rebels who carried out several bomb attacks on security forces.

Iran, a sharp critic of Bahrain's government, denies links to Bahrain's opposition. It does, however, champion their cause.

Tehran called the punishments "reckless".

"Bahrain's government has demonstrated that it does not seek a peaceful resolution and a way out out of the crisis," said Bahram Qasemi, the spokesman of Iran's foreign ministry, quoted by the official news agency IRNA.

Death sentences

The Bahraini high court on Monday upheld the death sentences against the trio convicted in a bomb attack in March 2014, which killed the policemen.

Seven other defendants received life terms.

The Emirati officer was part of a Saudi-led Gulf force which rolled into Bahrain in March 2011 to help security forces put down a month of protests led by the country's Shia majority.

Bahrain's majority Shia population has for decades accused their rulers of discrimination in matters of jobs, housing and political say.

Source: News agencies