The Pentagon has notified the U.S. Congress it plans to sell Saudi Arabia 15,000 anti-tank missiles worth more than $1.1 billion, a massive purchase of that type of weapon by any standard outside of wartime emergency.

It's not clear why the kingdom suddenly wants such a large amount of advanced anti-tank missiles, particularly since it currently possesses only some 4,000 anti-tank weapons of all types, according to the current Military Balance published by the International Institute for Strategic Studies in London.

Analysts suspect the Saudis plan to send most of the tube-launched anti-tank missiles they now have to bolster Syrian rebels that are supported by the kingdom at a time when the Free Syrian Army, considered a moderate, secular force, is in danger of being overwhelmed by increasingly powerful Islamist rivals and Iranian-armed regime forces.

The Saudis are prohibited by the Americans from giving U.S.-built weapons systems sold by Washington to the kingdom to anyone else. So any anti-tank weapons that go to the Syrian rebels will have to be drawn from Saudi Arabia's stocks of European weapons.

The U.S. Defense Security Cooperation Agency told Congress the Saudi order includes 9,650 BGM-71 2A optically-tracked, wire-guided missiles and 4,145 BGM-71 2B missiles and would be used to "improve Saudi Arabia's capability to meet and defeat current and future threats."

The journal Foreign Report quoted former U.S. Ambassador to Riyadh Charles Freeman as saying: "I would speculate that that with an order of this size, the Saudis are flushing them current stocks in the direction of the opposition and replacing them with new munitions."

If the Saudis are planning to provide the rebels with significant amounts of anti-tank missiles from existing stocks, they will need to get them to the FSA and its allies as soon as possible because these forces are under increasing pressure on and off the battlefield.

Right now, the Syrian regime, backed by Iran and thousands of fighters from Lebanon's Hezbollah and Iranian-trained Iraqi Shiite groups, is conducting a major offensive in the mountainous Qalamoun region.

These forces are pushing westward from Damascus to the Lebanese border to smash rebel bases and seize control of the key M5 highway to link Damascus with the Mediterranean ports of Latakia and Tartous.

Securing these objectives would immensely strengthen the regime's hand at planned peace talks scheduled in Geneva in mid-January, and probably ensure that President Bashar Assad would remain the Syrian leader in any peace settlement.

That's anathema to the Saudis, who want Assad, a key Arab ally of Iran, toppled and preferably killed.

So Riyadh may now believe it's a race against time to hit Assad's forces hard before the Damascus regime can notch up a new battlefield victory.

The way the Saudis see things, the U.S. efforts to effect a reconciliation with Iran and scale down Tehran's contentious nuclear program, at a time when Washington's eyes are increasingly on the threat from China, leave them isolated.

U.S. President Barack Obama's refusal to take military action against Assad's regime following an Aug. 21 chemical attack that reportedly killed 1,400 Syrian civilians -- apparently because back-channel negotiations with Iran were nearing fruition -- incensed the Saudis and their gulf allies.

This exposed longtime differences between Washington and Riyadh. The Obama administration is now seeking to limit the political damage by offering Riyadh advanced weapons systems that the U.S. has withheld for years, largely because of concerns they could be used against Israel some day.

Mounting Saudi frustrations, particularly over events in Syria, were voiced Saturday by Prince Turki bin Faisal, formerly head of Saudi Arabia's General Intelligence Directorate and later ambassador to the United States.

He accused the United States and Britain of refusing to "provide the necessary aid" to the FSA "to allow it to defend itself and the Syrian people from Assad's killing machine."

Since the Syrian war began in March 2011, Riyadh has provided the rebels with anti-tank weapons purchased in Croatia and smuggled to the FSA in northern Syria, as well as Chinese HJ-8 missiles funneled to them through Jordan in the south.

The rebels have also seized dozens of Soviet-era 9M113 Konkurs weapons from Syrian army bases they have overrun.

But they've never had enough of these weapons to compete with Assad's firepower.