Hours before arriving in Saudi Arabia on Tuesday, Turkish President Recep Tayyip Erdogan accused Syria's president of "mercilessly" killing hundreds of thousands of people and criticized Russia for backing him.

Erdogan was speaking to reporters before departing for Saudi Arabia, where he will meet King Salman for talks focused on the Syrian civil war. Turkey and Saudi Arabia are strong backers of the rebels fighting to overthrow Syrian President Bashar Assad, who in turn is supported by Russia and Iran.

Erdogan said his government and Saudi Arabia are working "in solidarity and consultation" to find a political solution for Syria, as both countries push for an agreement that would remove Assad from power.

In comments apparently directed at Russia's military intervention in Syria, Erdogan said: "You cannot go anywhere by supporting a regime that has mercilessly killed 400,000 innocent people with conventional and chemical weapons."

Russia began airstrikes in Syria on Sept. 30, saying it wanted to support the Syrian government and defeat Islamic State in Iraq and Syria (ISIS) militants and other extremists. But many of the strikes have hit Western-backed rebel groups in areas where ISIS is not present, and Syrian activists say the Russian strikes have killed civilians.

The UN says at least 250,000 people have been killed in the nearly five-year Syrian conflict, and some 12 million people displaced, triggering a massive refugee crisis. What began in 2011 as mainly peaceful protests inspired by the Arab Spring eventually spiraled into an armed conflict pitting rebels against the military, drawing in global powers as well as extremist groups like ISIS and al-Qaeda.

"Adding fuel to fire"

Though Kurdish Sunni fighters are among the strongest forces on the ground in Syria battling ISIS, Erdogan told reporters before arriving in Saudi Arabia that countries backing the Kurds are "adding fuel to fire." Turkey considers the Kurdish forces in Syria terrorists because of their links to an outlawed Kurdish rebel group in Turkey.

When Saudi Arabia hosted a meeting of major Syrian opposition groups this month, the main Kurdish militia known as the YPG and the largest Kurdish group, the Democratic Union Party or PYD, were not invited.

This is Erdogan's third visit to Saudi Arabia this year. He flew to Riyadh in January to attend the late King Abdullah's funeral and again in March for bilateral talks with King Salman. The two leaders also met at the G-20 summit held in Turkey last month.