Bahrain's Prime Minister Khalifa bin Salman Al Khalifa has passed away at the age of 84, the state news agency reported on Wednesday.

He breathed his last at a hospital in the United States on Wednesday morning, the report said. 

"The Royal Court mourns His Royal Highness ... who passed away this morning at Mayo Clinic Hospital in the United States of America," the Bahrain News Agency said, adding that the country would enter a week of official mourning. 

Khalifa was the world's longest-serving prime minister who had held the post since Bahrain's independence in 1971.

Bahrain's King Hamad announced one-week mourning with flags flown at half-mast.

During the mourning period, flags will be flown at half-mast and government ministries and departments closed for three days.

(With input from agencies)