Kuwait City, Jul 26 (IANS): Kuwaiti Crown Prince Sheikh Nawaf Al-Ahmad Al-Jaber Al-Sabah has sent a letter to US President Donald Trump thanking him for sending an American Air Force plane to transport Emir Sheikh Sabah Al-Ahmad Al-Jaber Al-Sabah to Washington for treatment, a media report said.

In the letter, Sheikh Nawaf also said the gesture represented the solid ties between Kuwait and the US, the Gulf News reported on Saturday.

The 91-year-old Emir left Kuwait on Thursday to complete his medical treatment in the US.

Hewas admitted to a hospital on July 18 for a medical checkup and had a "successful" surgery a day later.

The reason for the surgery or what treatment the Emir is receiving in the US has not been disclosed.

Crown Prince Sheikh Nawaf, the Emir's designated successor, was given some of the ruler's constitutional duties, temporarily.

When the Emir is absent, under Kuwaiti law, the Crown Prince is appointed as acting ruler.

Sheikh Nawaf was sworn in as Crown Prince in 2006 after Sheikh Sabah was appointed Emir.

Last year, the Emir was reported to have suffered a health "setback" while in Kuwait.

The following month, he was hospitalised for medical tests during a visit to the US.

Sheikh Sabah has been credited with shaping the Gulf nation's foreign policy and spent decades as foreign minister before becoming emir in 2006.