The UN Security Council on Monday backed a Russian initiative to bar trade in oil with Islamists in Iraq and Syria.

The 15-nation Council warned in a joint statement that buying oil from groups such as the Islamic State and Jabhat Al-Nusra fighting in Iraq and Syria could lead to sanctions.

"Such engagement constitutes financial support for terrorists and may lead to further sanctions listings," the council said.

Russia, a close ally of Syrian President Bashar al-Assad, presented the statement in late June, seeking to clamp down on middle-men who are selling the oil from Islamist-controlled areas.

Moscow's Ambassador Vitaly Churkin said he was pleased with the stance, saying it was a "reminder" from the top world body that "this amounts to a very serious transgression" of UN resolutions.

Islamic groups such as ISIL, which rebranded itself as the Islamic State, and Al-Nusra have seized oilfields and pipelines to bankroll their offensives.

The council said control of oil facilities "could generate material income for terrorists, which would support their recruitment efforts, including of foreign terrorist fighters, and strengthen their operational capability to organize and carry out terrorist attacks."

Trade of oil with the Islamists is in violation of UN resolutions and "all states are required to ensure that their nationals and any persons within their territory do not trade in oil with these entities," it said.

The Syrian Observatory of Human Rights said last week that Islamic State (IS) fighters in Syria were selling oil and gas to Iraqi businessmen.

IS insurgents took control of large parts of Iraq's north and west in a sweeping offensive that began on June 9, preventing Baghdad from exporting oil via a pipeline to Turkey and by road to Jordan.