An unnamed Middle Eastern state has supplied Syrian rebels with 250 sophisticated Soviet-made anti-tank missiles, most of which were given to radical Islamist militias fighting President Bashar Assad, according to a report published in London-based Arabic daily A-Sharq Al-Awsat on Tuesday.

According to the report, the unidentified state made its first delivery of 9M113 Konkurs missiles to the rebels a week ago via Turkey. The Konkurs antitank missile has a maximum range of four kilometers and a hit probability of 90 percent. Egypt, Iran and Turkey are the only known operators of the missile system in the Middle East.

Videos uploaded to YouTube in the past two months claim to show rebels using Konkurs missiles to destroy Assad’s armor.

The Konkurs missile was responsible for a portion of the 50 Merkava tanks damaged by Hezbollah during the 2006 Second Lebanon War, according to a Defense Ministry report published by Globes shortly after the conflict. According to Merkava tank program administration figures cited by the paper, Hezbollah’s Russian-made RPG 29, Kornet E, Metis-M, and Konkurs missiles penetrated 22 tanks during the monthlong conflict, killing 23 IDF crewmen.

Tuesday’s report was the latest in a series of unconfirmed reports of Arab states providing the Syrian opposition with sophisticated weaponry to fight Assad’s better-equipped military. German newspaper Der Spiegel reported Sunday that Saudi Arabia sought to arm the rebels with European-made shoulder-launched anti-aircraft missiles, and Reuters quoted a Gulf source saying Riyadh has been providing the weapons for two months already.

US President Barack Obama last week decided to send the rebels weapons and ammunition for the first time in an attempt to increase their military strength and bolster their political bargaining power. But the American inventory for the rebels is not yet expected to include the high-powered weaponry sought by the opposition, raising questions about whether the deepening US involvement will be effective in changing the situation on the ground.

The White House also announced Monday an additional $300 million in humanitarian aid for Syria and neighboring countries absorbing refugees escaping the violence. The new money brings the total US humanitarian assistance to $800 million, according to the White House.

Assad warned Europe in a interview published on Monday that it “would pay a price” if it delivered weapons to rebels fighting to topple him, and that arming them would backfire as the “terrorists” return to their countries with extremist ideologies.