Jordan’s King Abdullah travelled to Germany on Sunday for urgent surgery to his spine, official media said.
Crown Prince Hussein, 27, took over as the regent during his father’s absence from the country.
“His majesty left the homeland to Germany for surgery on a slipped disc in the spinal chest region,” state television said
“His Highness Prince Hussein, the crown prince, took the constitutional oath as regent to king, in the presence of the Cabinet,” the broadcast said.
The palace said on Saturday that the king, who is 60, needed “urgent” back surgery owing to pain from a slipped disc, and that his doctors advised him to have the operation in Frankfurt.
The king has had back problems for years after military service, before he succeeded his father, the late King Hussein, in 1999.
State media said his back injury was related to his military service, which has included extremely physical activity including parachuting.
He is expected to remain in Germany for a week, it said.
His son, the crown prince, has been given more assignments in the past two years and was promoted in the military.
The king has also been taking Prince Hussein, 27, with him on more trips abroad.
When the two men are abroad, King Abdullah usually appoints his younger brother, Prince Faisal, as the regent, the person who takes over regal duties in the monarch’s absence.
King Abdullah was last in Germany on March 15 on an official visit to meet Chancellor Olaf Scholz.
The kingdom of 10 million people is in a severe economic downturn, with unemployment officially hovering around 24 per cent. It is one of the largest recipients of US aid and signed last year a military pact with Washington.
In the past month King Abdullah has been on a diplomatic drive to contain Palestinian-Israeli tension, travelling to Ramallah to meet Palestinian President Mahmoud Abbas at the end of March.
He also met several senior Israeli officials in Amman in the past few weeks.