RIYADH (AP) -- Saudi Arabia's newly enthroned King Salman moved quickly Friday to name a future successor to the crown in his oil-rich kingdom, a significant appointment that puts the kingdom's future squarely in the hands of a new generation.

King Salman bin Abdul-Aziz Al Saud's actions came as the Sunni-ruled kingdom mourned King Abdullah, who died early Friday at the age of 90 after nearly two decades in power.

A royal decree affirmed Crown Prince Muqrin as his immediate successor. After Muqrin, Salman named Interior Minister Prince Mohammed bin Nayef as deputy crown prince, making him second-in-line to the throne. Mohammed is the first grandson of Saudi Arabia's founder, King Abdul-Aziz Al Saud, to be named as a future heir.

King Salman promised in a nationally televised speech to continue the policies of his predecessors.

We will continue adhering to the correct policies which Saudi Arabia has followed since its establishment,'' Salman said.

For more than six decades, power has passed among the sons of King Abdul-Aziz, from brother to brother, since his death in 1953. But ranks of that generation, largely in their 70s and 80s, are thinning.

The decision to name Mohammed as deputy crown prince helps alleviate uncertainty over which of Abdul-Aziz's hundreds of grandsons would ascend to the throne.

Salman on Friday also appointed his son, Prince Mohammed, as Defense Minister. The prince, in his 30s, was head of his father's royal court when Salman was crown prince and is among his most-favored sons.

In his first speech as king, Salman also made an oblique reference to the chaos gripping the greater Middle East as the extremist Islamic State group now holds a third of both Iraq and Syria.

The Arab and the Islamic nations are in dire need of solidarity and cohesion,'' the king said.

Salman, 79, had increasingly taken on the duties of the king over the past year as his ailing predecessor and half brother, Abdullah, became more incapacitated. Abdullah officially ascended to the throne in 2005, but had been de-facto ruler for years before that.

Abdullah is being buried Friday afternoon following a funeral at the Imam Turki bin Abdullah mosque in the capital, Riyadh. Muslim dignitaries from around the world began arriving in Riyadh for the funeral, under heavy guard.

State television aired images of the prayer ahead of his burial, which showed King Abdullah's body shrouded in a simple beige cloth in line with Islamic tradition.

Leaders from around the world expressed their condolences for King Abdullah.

U.S. President Barack Obama described the late Saudi king as a candid leader who had the courage of his convictions, including his steadfast and passionate belief in the importance of the U.S.-Saudi relationship as a force for stability and security in the Middle East and beyond.''

The president of the neighboring United Arab Emirates, Sheikh Khalifa bin Zayed Al Nahyan, said in a statement that Abdullah generously gave a lot to his people and his nation,'' while Egyptian President Abdel-Fattah el-Sissi said the Saudi kingdom and the Arab nation have lost a leader of its best sons.''

Salman served as defense minister since 2011. That made him the head of the military as Saudi Arabia joined the United States and other Arab countries in carrying out airstrikes in Syria in 2014 against the Islamic State group, the Sunni militant group that the kingdom began to see as a threat to its own stability.

Salman takes the helm at a time when the ultraconservative Muslim kingdom and oil powerhouse is trying to navigate social pressures from a burgeoning youth population _ over half the population of 20 million is under 25 _ seeking jobs and increasingly testing boundaries of speech on the Internet, where criticism of the royal family is rife. The country is also facing plunging global oil prices, which forms the backbone of its economy.

Salman's health has been a question of concern. He suffered at least one stroke that has left him with limited movement on his left arm.

Salman is among the so-called Sudeiri Seven'' _ seven sons born to one of Abdul-Aziz's most favored wives, Hussa bint Ahmad Sudeiri. The seven brothers were seen as a center of power within the family. Abdullah's predecessor, King Fahd, was among the seven, as were Abdullah's first two crown princes, Sultan and Nayef, who died in 2011 and 2012 respectively before reaching the throne.

Prince Mohammed is the son of Salman's brother Nayef. Like his father, Nayef, who was a formidable power in Saudi Arabia until his death in 2012, Mohammed is head of the powerful Interior Ministry that oversees the country's police. Mohammed was the target of a botched assassination attempt by al-Qaida militants in 2009.