DUBAI - Saudi Arabia's King Salman said on Sunday he was "heartbroken" over a suicide bombing at a Shi'ite mosque in the kingdom that killed 21 people, state news agency SPA reported, a conciliatory statement as sectarian strife intensifies in the region.Salman said anyone linked to the attack, claimed by the Islamist militant group Islamic State, or who sympathizes with it, will be brought to justice."We were pained by the enormity of the crime of this terrorist aggression which contradicts Islamic and humanitarian values," the king said in a message to Crown Prince Mohammed bin Nayef, who is also the interior minister."Any participant, planner, supporter or sympathizer with this heinous crime will be held accountable, tried and will receive the punishment he deserves," he said.The bombing in Saudi Arabia came as tensions between Sunni and Shi'ite Muslims are on the increase in the region. Some clerics in Saudi Arabia, the birthplace of Islam and mainstay of its Sunni denomination, are deeply hostile towards Shi'ites, whom they regard as apostates.A Sunni militant blew himself up in the al-Qadeeh village mosque in Saudi Arabia's heavily Shi'ite east during Friday prayers, in one of the worst attacks in the kingdom in years.The Saudi Interior Ministry identified the perpetrator as Saleh bin Abdul Rahman Saleh Qashimi, a Saudi citizen wanted for belonging to Islamic State. The militant group had identified him on Friday by a nom de guerre, Abu 'Ammar al-Najdi. Saudi Arabia is part of an international coalition carrying out bombing strikes on Islamic State targets in Iraq and Syria.Saudi and Gulf governments fear that sectarian wars in Yemen, Syria and Iraq where they and other Sunni Muslim allies are fighting groups close to Shi'ite arch-rival Iran will radicalize their citizens and harm domestic security.