Militant sources told the British newspaper Daily Telegraph on Wednesday that the terrorists had been supplied with Russian-made “Konkurs” anti-tank missiles.

“A handful of the missiles were already in use and in high demand after opposition forces looted them from captured regime bases,” the report said.

More have now arrived, confirming reports that the White House has lifted an unofficial embargo on its Persian Gulf allies sending heavy weapons to the militants, it added.

Last week, the White House said it would send military support to Syria’s opposition after accusing that that the Syrian government had used chemical agents against the foreign-backed militants.

Unlike rocket-propelled grenades, the Konkurs – Contest in English – can penetrate the most advanced tanks such as Russian-made T72s.

“We now have supplies from Saudi Arabia,” a terrorist source said. “We have been told more weapons are on their way, even higher-end missiles.”

At the G8 this week Vladimir Putin, the Russian president, condemned the West’s attempts to send arms to the militants.

Putin warned that those in the west who want to arm Syria insurgents may find the weapons ended up anywhere, even in Europe.

"We are calling on all our partners to once again think thoroughly before taking this very dangerous step,” he told reporters after a G8 summit in Northern Ireland.

"Europe wants to send arms to these people? And what happens then to these arms? Who will control in whose hands and where they will end up? Maybe in Europe?"

He also rejected US and European claims that Syria's government had used chemical weapons.

"We do not have facts pointing to the use of such weapons by the Syrian government," Putin said.

Syria has been gripped by a deadly unrest since March 2011, and thousands people, including large numbers of government forces, have been killed in the violence.