FILE PHOTO: Kuwait's Crown Prince Sheikh Nawaf al-Ahmad al-Sabah waves as he arrives for the opening session of Parliament in Kuwait City December 16, 2012. REUTERS/Stephanie Mcgehee

RIYADH (Reuters) - Kuwait’s Crown Prince Sheikh Nawaf al-Ahmad al-Sabah traveled to the United States for “routine” medical tests on Tuesday, state news agency KUNA reported.

Sheikh Nawaf, 80-year-old brother of the small Gulf Arab state’s ruler, underwent minor back surgery in 2013 in Germany. A relatively low-profile figure, he became next in line to lead the U.S.-allied oil exporter in 2006, shortly after Sheikh Sabah al-Ahmad al-Sabah became ruling emir.

The crown prince underwent successful medical check-ups last August in the United States, KUNA reported at the time.

The health of leading members of the ruling family is a sensitive issue in Kuwait, where less senior al-Sabah members have long been jostling for position, according to diplomats and analysts. It is not public knowledge who is third in line in Kuwait, an OPEC member state.