Russia is accusing three Turkish organizations of smuggling military equipment to the Islamic State group and other militant groups in Syria, in a letter to the UN Security Council released Friday.

"The main supplier of weapons and military equipment to ISIL fighters is Turkey, which is doing so through non-governmental organizations," wrote Russian Ambassador Vitaly Churkin in the letter dated March 18.

Churkin said the Besar foundation, the Iyilikder foundation and the Foundation for Human Rights and Freedoms had sent convoys of "various supplies" to the armed groups.

Russia has repeatedly accused Turkey of backing Islamist groups opposed to Syrian leader Bashar al-Assad, Moscow's ally in the five-year war.

The attacks on Turkey have grown more virulent since Ankara shot down a Russian warplane in November.

In his letter, Churkin argued that the three organizations were a front for Turkish intelligence operations in Syria.

Russia also named two companies -- Tevhid Bilisim Merkezi and Trend Limited Sirketi, both based in Sanliurfa -- that it said provided chemical components or detonating fuses to fighters in Syria.

In February, Russia failed to win UN Security Council backing for a draft resolution calling on Turkey to halt cross-border shellings in Syria.