US-BACKED Syrian forces seized the country’s largest oil field near the Iraqi border from Isis yesterday after the extremists allegedly switched sides, giving up the fields without a fight.

The Syrian Democratic Forces (SDF) — a predominantly Kurdish group which last week declared the “total liberation” of Isis’s former stronghold of Raqqa — said it had taken the al-Omar oil field northeast of Deir Ezzor in a “swift and wide military operation.”

But Lebanon’s Al-Masdar News claimed local militia who had pledged allegiance to Isis in 2014 had defected to the SDF, handing them the lucrative resource without a fight.

The British-based Syrian Observatory for Human Rights said government troops moving to retake the field before the SDF were forced to retreat after they came under artillery fire from Isis.

The al-Omar field produces an estimated 7,500 to 9,000 barrels a day of high-quality crude oil, and was a major source of revenue for Isis — until the Russian air force wiped out their fleet of thousands of tankers.

Meanwhile, Russia yesterday accused the US-led coalition in Syria of wiping Isis’s former “capital” Raqqa “off the face of the Earth” with carpet-bombing, comparing the damage to the US and Britain’s bombing of German’s Dresden during World War II.