British Prime Minister David Cameron said Monday tensions between Iran and Saudi Arabia were "hugely concerning" and condemned the death penalty following Riyadh's controversial execution of a Shiite cleric.

Sheikh Nimr al-Nimr was put to death on Saturday with 46 other people, setting off a chain of events which has seen Saudi Arabia and Bahrain severing their ties with Iran after protestors torched the Saudi embassy in Tehran and a consulate in second city Mashhad.

The crisis has raised fears of increased sectarian violence in the Middle East.

"It is hugely concerning because of course we want to see stability in the Middle East... not least because that will be absolutely essential for solving the crisis in Syria which is the source of so many of these problems," Cameron said on a visit to east London.

"We condemn and do not support the death penalty in any circumstances and that includes Saudi Arabia... we always make representations on the death penalty and the foreign office ministers made it very clear on this occasion."

Saudi Arabia is Britain's most important trade partner in the Middle East and was its biggest market for arms exports in 2014.

Nimr had been branded an "instigator of sedition" by Saudi Arabia. He was notably a force behind 2011 anti-government protests in the east of the country, where Shiites have long complained of marginalisation.

Executions have soared in Saudi Arabia since King Salman ascended the throne a year ago with 153 people put to death in 2015, nearly twice as many as in 2014.
Source: AFP