Russia is accusing three Turkish foundations of supplying weapons and military equipment to the Islamic State group in Syria and says $1.9 million worth of explosives and chemicals were smuggled across Turkey's border to extremist groups.

Russia's U.N. Ambassador Vitaly Churkin said in a letter to the U.N. Security Council circulated Friday that Turkey is the main supplier of weapons and ammunition to IS fighters. He said the transfers are overseen by the country's National Intelligence Organization.

Turkey's U.N. Mission did not immediately answer calls seeking comment on the Russian letter.

Russia's recent air campaign has helped close ally Syrian army make broad advances on the ground. Turkey, Saudi Arabia and other regional players have backed opponents of Syrian President Bashar Assad and oppose any strengthening of the Damascus government.