The death toll from a shooting near a Shiite mosque in Oman's capital has climbed to six, police said Tuesday, adding that the three attackers were also killed. 

Monday's shooting "resulted in the death of five individuals, the martyrdom of one policeman, and the death of the three perpetrators," with at least 28 people from various nationalities wounded, Omani police said in a statement.

Four Pakistani nationals were among the deceased and those injured.

Pakistani Prime Minister Shehbaz Sharif said on Tuesday that he's saddened by the terrorist attack, which has yet to be claimed, and has instructed the Pakistan Embassy in Muscat to extend all possible assistance to the injured and visit the hospitals personally. 

"Pakistan stands in solidarity with the Sultanate of Oman & offers full assistance in the investigation," he wrote in a social media post.

The attack came as Shi'ite Muslims observed Ashura, an annual period of mourning to commemorate the 7th-century death of Hussein, a grandson of the Prophet Muhammad. The observation of Ashura has occasionally triggered sectarian tensions between Sunni and Shi'ite Muslims in some countries, though not typically in Oman.

(With input from agencies)