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

Russia's UN Ambassador Vitaly Churkin said in a letter to the UN 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 and are delivered mainly by vehicles, "including as part of humanitarian convoys."

In an email, a spokesman for Turkey's UN Mission said the Russian letter "obviously contains baseless allegations which we totally reject." He pointed out that many Turkish citizens have been killed in attacks by IS, which he called a national security threat.

Russia's air campaign that began on Sept. 30 has helped close ally Syria make broad advances on the ground in the five-year-old war that has killed over 250,000 people. Turkey, Saudi Arabia and other regional players have backed opponents of Syrian President Bashar Assad and are loath to see him strengthen his grip on power.

The Turkish spokesman said Russia's letter was "an attempt to overshadow the civilian deaths, havoc and destruction in Syria caused by the military operations of the Russian Federation, the regime's staunchest ally."

The Islamic State group is on the back foot in Iraq and Syria, where forces on the ground supported by U.S.-led airstrikes have targeted the extremists. Russian aircraft are also targeting IS positions. The U.S.-led international coalition estimates that the militant group has lost 40 per cent of the territory it once held in Iraq and around 20 per cent of its territory in Syria.

Churkin's letter gave details of convoys with military equipment and munitions for IS fighters, and of funding allegedly arranged by the three foundations. It said one foundation has sent 7,500 vehicles with various supplies to IS-controlled territory since 2011.

The letter said that among the supplies delivered to IS were ammunition for TOW anti-tank missile systems, RPG-7 grenade launchers and small arms, M-60 recoilless rifles, 82mm mortar shells, hand grenades, communication tools and equipment from the Turkish intelligence services.

The letter also said "smuggling explosives and industrial chemicals to terrorist groups operating in Syria is also usually organized from Turkish territory" through the border crossings. "In order to pass through the border controls unimpeded, effectively with the complicity of the Turkish authorities, products are processed for companies that are purportedly registered in Jordan and Iraq," it said.

Churkin said large consignments of explosives are often transported by water, especially the Euphrates River.

The letter listed the total supplies of explosives and industrial chemicals sent "to terrorists through Turkey in 2015." It said 2,500 tons of ammonium nitrate worth approximately $788,700 were sent along with 456 tons of potassium nitrate worth $468,700, 75 tons of aluminum powder worth $496,500, sodium nitrate worth $19,400, glycerin worth $102,500, and nitric acid worth $34,000.