Saudi Arabia's King Abdullah bin Abdulaziz
Saudi Arabia's King Abdullah bin Abdulaziz al-Saud attends a meeting at the Saudi Royal palace in Riyadh on Dec. 29, 2013. Reuters/Kenzo Tribouillard

Saudi Arabia’s King Abdullah bin Abdulaziz was admitted to the National Guard Hospital in Riyadh for medical tests, CNN reported, citing an official statement, which did not give further details on the monarch's health.

Abdullah, 90, was named the crown prince in 1982 and took over as the leader of Saudi Arabia in 2005, succeeding King Fahd. Abdullah’s half brother Salman, 77, who was named crown prince in June 2012 after the death of Prince Nayef bin Abdul Aziz, is next in line to ascend the throne, Agence France-Presse reported. Earlier this year, Abdullah had also appointed Prince Muqrin bin Abdulaziz as the deputy crown prince, Reuters reported.

"The Custodian of the Two Holy Mosques, King Abdullah bin Abdulaziz, may God keep him, entered today Wednesday ... the King Abdulaziz Medical City of the National Guard in Riyadh to undergo some medical tests," according to the Saudi Press Agency, cited by Reuters.

Under Abdullah's leadership, the country has begun providing basic freedoms to broader groups of people, including women, and has also invested some of its oil wealth in large-scale education and infrastructure projects, CNN reported. However, some conservative groups in the country oppose the freedom being demanded for women, including the permission to drive. The country, which is the world's largest exporter of oil, is a major U.S. ally in the region.

The Saudi stock market, which had already dipped over 1 percent due to falling oil prices, dipped a further 5 percent after the news of the king's hospitalization, Reuters reported, adding that markets recovered slightly in later trading.

Abdullah has undergone surgeries in the past related to a herniated disc. In 2012, he underwent an 11-hour operation at the same Riyadh hospital, and had the same operation once before in 2011, and, in 2010, had two surgeries on his back in the U.S., Reuters reported.