Â鶹ӰÊÓ

WASHINGTON -- CIA-backed rebels in Syria, who had begun to put serious pressure on President Bashar Assad's forces, are now under Russian bombardment with little prospect of rescue by their American patrons, U.S. officials say.

Over the past week, Russia has directed parts of its air campaign against U.S.-funded groups and other moderate opposition in a concerted effort to weaken them, the officials say. The Obama administration has few options to defend those it had secretly armed and trained.

The Russians "know their targets, and they have a sophisticated capacity to understand the battlefield situation," said Republican Rep. Mike Pompeo, who serves on the House Intelligence Committee and was careful not to confirm a classified program. "They are bombing in locations that are not connected to the Islamic State" group.

Other U.S. officials interviewed spoke on condition of anonymity because they were not authorized to discuss the matter publicly.

The CIA began a covert operation in 2013 to arm, fund and train a moderate opposition to Assad. Over that time, the CIA has trained an estimated 10,000 fighters, although its current size isn't clear.

The effort was separate from the one run by the military, which trained militants willing to promise to take on IS exclusively. That program was widely considered a failure, and on Friday, the Defence Department announced it was abandoning the goal of a U.S.-trained Syrian force, instead opting to equip established groups to fight IS.

For years, the CIA effort had foundered -- so much so that over the summer, some in Congress proposed cutting its budget. Some CIA-supported rebels had been captured; others had defected to extremist groups. The secret CIA program is the only way the U.S. is taking on Assad militarily. In public, the United States has focused its efforts on fighting IS and urging Assad to leave office voluntarily.

"Probably 60 to 80 per cent of the arms that America shovelled in have gone to al-Qaida and its affiliates," said Joshua Landis, a Syria expert at the University of Oklahoma.

But in recent months, CIA-backed groups, fighting alongside more extremist factions, began to make progress in Syria's south and northwest, American officials say. In July and August, U.S.-supported rebels seized territory on the al-Ghab plain, in northwest Syria's Idlib and Hama governorates. The plain is a natural barrier between areas controlled by Sunni Muslims and the Alawite sect to which Assad and his loyalists belong. The capture of the al-Ghab plain was seen as a breakthrough toward weakening the Alawites.

Those and other gains put Damascus, the capital, at risk, officials say.

But in recent days, Russian airstrikes have hit groups in the area, according to the Institute for the Study of War, a Washington think-tank that closely tracks the situation. Russian bombs and missiles have hit specific buildings associated with the moderate Syrian opposition, according to a U.S. official briefed on the intelligence.

Russian officials have insisted they are bombing Islamic State militants and other terrorists.

U.S. intelligence officials see many factors motivating Russia's intervention: Moscow's reasserting its primacy as a great power, propping up Assad and wanting to deal a blow to the United States, which has insisted that Assad must go to end Syria's civil war.

Russia is also interested in containing IS, an organization that includes thousands of Chechen fighters who may pose a threat to Russia, officials say.

But in the short term, "my conclusion is that the timing of their intervention was driven by Assad really going critical," said Democratic Rep. Jim Himes, also a House Intelligence Committee member.

The administration is scrambling to come up with a response to Russia's moves, but few believe the U.S. can protect its secret rebel allies. The administration has all but ruled out providing CIA-backed groups with surface-to-air missiles that can down aircraft, fearing such weapons would end up in the wrong hands, officials say.

Rep. Adam Schiff, the top Democrat on the committee, says the U.S. should consider establishing a no-fly zone that allows rebels a safe place from which to operate, and shooting down Syrian helicopters that are bombing civilians. He said the U.S. also should provide arms to the Ukrainian government fighting Russian-backed separatists.

A no-fly zone would require the U.S. military to be ready to engage in air battles with the Syrian government, something it is not prepared to do.

The administration "is debating the merits of taking further action or whether they are better off letting Putin hang himself," he said, referring to Russian President Vladimir Putin.

"Our options are much narrower than they were two weeks ago," said Independent Sen. Angus King, who serves on the Intelligence and Armed Services committees. "I don't think there is any simple answer. ... Further air involvement has become very problematic because of the Russian engagement."