Soldiers from 20 countries are to gather in Saudi Arabia for massive military exercises lasting 18 days, the official Saudi Press Agency (SPA) said. It comes as Riyadh has openly warned Syrian President Bashar Assad that he will be toppled.

The Saudi state agency made the announcement on Sunday, adding that participating troops will begin arriving in “the next few hours.”

The oil-rich nation described the exercises as “the largest and most important” military drills in the region’s history.

The so-called “Northern Thunder” exercise will take place in the north of the country and will include air, sea and land forces. SPA said that it will show that Riyadh and its allies “stand united in confronting all challenges and preserving peace and stability in the region.”

Among the participants will be Arab and African countries. The US and other Western powers have not been invited.

BREAKING: Saudi jets to fly missions in Syria from Turkish base https://t.co/w0CK8KzUiSpic.twitter.com/VCqbJFyEqN — RT (@RT_com) February 13, 2016

Sunday’s announcement comes as Saudi Arabia, which is a member of the US-led coalition against Islamic State (IS, formerly ISIS/ISIL), deployed military jets and personnel to Turkey’s Incirlik Air Base. The base is already used by the US Air Force for their planes conducting sorties in Syria.

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While Riyadh says this necessary to “intensify” its operations against Islamic State in Syria, the move has sparked concern that the Saudis are getting ready for a full-scale ground invasion of war-torn Syria, where they are backing anti-government rebels battling Syrian President Assad.

In a recent interview with American media, the Saudi Foreign Minister flatly stated that Assad will be toppled if he does not leave during a political transition.

“Bashar al-Assad will leave – have no doubt about it. He will either leave by a political process or he will be removed by force,” Adel al-Jubeir told CNN.

At the same time, Saudi Arabia is leading a coalition that has been bombing Houthi rebels in Yemen since March. Riyadh went to war in Yemen to restore ousted President Abd Rabbuh Mansur Hadi, who fled from the Shiite Houthi rebels after his two-year term expired in January.

His predecessor, Ali Abdullah Saleh, who was formerly an opponent of the rebels, is now their ally and provides assistance them with tribal troops loyal to him.

READ MORE: Turkish, Saudi troops in Syria would be ‘catastrophic’ (OP-ED)

The Saudis also created a 35-member coalition to battle “terrorism” in Muslim countries last December.