Saudi Arabia's King Abdullah has died, a statement from the royal palace has said.

The royal court said in a statement on Friday that Abdullah, believed to be around 90, died at 1:00am local time (2200 GMT), expressing its "great sadness and mourning".

The officials did not disclose the cause of Abdullah's death, but he was admitted to hospital in December suffering from pneumonia and had been breathing with the aid of a tube.

Al Jazeera's Hashem Ahelbarra said: "This is someone who is definitely going to be remmebered as a reformist within the royal family.

"He succeeded his brother at a very delicate time and started reforms in the country.

Crown Prince Salman bin Abdulaziz al-Saud, deceased king's half-brother, has officially been named as the next ruler of the world's top oil exporter and the spiritual home of Islam. Prince Muqrin Bin Abdulaziz is the new crown prince, Saudi state TV announced.

Saudi Arabia's Western and Arab allies, along with countries such as Israel and China, have offered their condolences on the death of the king.

Abdullah will be buried later on Friday following afternoon prayers, and citizens would be invited to pledge allegiance to the new monarch and the crown prince at the royal palace, the royal court statement said.

King Salman, 79, in his first public address, pledged no change in the kingdom's direction and called for unity among Muslims.

"We will remain with God's strength attached to the straight path that this state has walked since its establishment by King Abdul Aziz bin al-Saud, and by his sons after him," Salman said in televised remarks.

The new King appointed his nephew Mohammed bin Nayef as deputy crown prince, meaning he will be the first of that generation to rule the kingdom one day.

Prince Mohammed, who remains as Interior Minister, according to the royal decree carried on state television, is next in line to rule after Salman and Crown Prince Muqrin.

Obama condolences

US President Barack Obama offered his condolences on King Abdullah's death.

"As our countries worked together to confront many challenges, I always valued King Abdullah's perspective and appreciated our genuine and warm friendship," Obama said in a written statement.

"The closeness and strength of the partnership between our two countries is part of King Abdullah's legacy."

Abdullah, thought to have been born in 1923, took the throne in 2005, but had run the country as de facto regent for a decade before that after his predecessor King Fahd suffered a debilitating stroke.

At stake with the appointment of Salman as king is the future direction of the United States' most important Arab ally and self-appointed champion of Sunni Islam at a moment of unprecedented turmoil across the Middle East.

Most recently the kingdom joined the US-led coalition carrying out air strikes against the Islamic State of Iraq and Levant.

Abdullah played a guiding role in Saudi Arabia's support for Egypt's government after the military intervened in 2013, and drove his country's support for Syria's rebellion against President Bashar al-Assad.

King Salman has been crown prince since 2012 and has been heading the defence ministry since 2011. He was governor of Riyadh province for five decades before that.