Saudi Arabia signed on Thursday preliminary agreements to buy S-400 air defense systems and receive "cutting edge technologies" from Russia during King Salman's landmark visit to Moscow, the Saudi military industries firm said.

The agreement was announced as King Salman, who is on the first official trip to Russia by a Saudi monarch, and Russian President Vladimir Putin held talks at the Kremlin.

Saudi Arabia's King Salman, on a landmark visit to Russia, and President Vladimir Putin on Thursday signed a slew of arms and energy deals as the key U.S. ally seeks to deepen cooperation with Moscow.

Saudi Arabia's decision to allow women to drive had some sounding the death knell for ride-hailing apps like Careem, but its co-founder expects business to flourish and even plans to hire thousands of female drivers.

Jubilant soccer fans celebrated after Saudi Arabia qualified for next year's World Cup, with many rallying behind Crown Prince Mohammed bin Salman's reform program that promotes sports in the ultra-conservative kingdom.

Iran and Saudi Arabia will soon exchange diplomatic visits, Tehran said, in a possible sign of tensions easing after the archrivals cut ties last year.

Closer ties between Saudi Arabia and U.S. President Donald Trump's administration helped pave the way for a succession shake-up making a 31-year-old prince the kingdom's de facto ruler, analysts say.

On Wednesday King Salman, 81, named his son Mohammed bin Salman crown prince and heir to the throne after firing Mohammed bin Nayef, whose counter-terrorism expertise had made him a favorite of previous American administrations.

The Al-Saud dynasty gave its family name to the Gulf Arab monarchy of Saudi Arabia ruled by the sons of founder Abdul Aziz bin Saud.

Saudi King Salman, who ascended the throne in January 2015 and the last of the "Sudairi Seven" to rule, has presided over wideranging efforts to transform his oil-dependent, conservative Islamic realm.

Saudi Arabia's hard-charging Mohammed bin Salman, named crown prince on Wednesday, holds unusual power for a man of just 31, so much so that diplomats nickname him "Mr Everything".

The son of King Salman has risen to become Saudi Arabia's most influential and prominent figure since being named second-in-line to the throne in early 2015.

Saudi Arabia and Iraq have made a "qualitative leap" in relations, they said Tuesday after a visit by Prime Minister Haider al-Abadi that followed years of strained ties.